.bp 13 .tc 6.6 The BIOS Entry Points .he CP/M Operating System Manual 6.6 BIOS Entry Points .sh 6.6 The BIOS Entry Points .qs .pp 5 The entry points into the BIOS from the cold start loader and BDOS are detailed below. Entry to the BIOS is through a jump vector located at 4A00H+b, as shown below. See Appendixes A and B. The jump vector is a sequence of 17 jump instructions that send program control to the individual BIOS subroutines. The BIOS subroutines might be empty for certain functions (they might contain a single RET operation) during reconfiguration of CP/M, but the entries must be present in the jump vector. .pp The jump vector at 4A00H+b takes the form shown below, where the individual jump addresses are given to the left: .mb 4 .fm 1 .mt 4 .hm 1 .sp 2 .nf .in 5 4A00H+b JMP BOOT ;ARRIVE HERE FROM COLD START LOAD .sp 4A03H+b JMP WBOOT ;ARRIVE HERE FOR WARM START 4A06H+b JMP CONST ;CHECK FOR CONSOLE CHAR READY 4A09H+b JMP CONIN ;READ CONSOLE CHARACTER IN 4A0CH+b JMP CONOUT ;WRITE CONSOLE CHARACTER OUT 4A0FH+b JMP LIST ;WRITE LISTING CHARACTER OUT 4A12H+b JMP PUNCH ;WRITE CHARACTER TO PUNCH DEVICE 4A15H+b JMP READER ;READ READER DEVICE 4A18H+b JMP HOME ;MOVE TO TRACK 00 ON SELECTED DISK 4A1BH+b JMP SELDSK ;SELECT DISK DRIVE 4A1EH+b JMP SETTRK ;SET TRACK NUMBER 4A21H+b JMP SETSEC ;SET SECTOR NUMBER 4A24H+b JMP SETDMA ;SET DMA ADDRESS 4A27H+b JMP READ ;READ SELECTED SECTOR 4A2AH+b JMP WRITE ;WRITE SELECTED SECTOR 4A2DH+b JMP LISTST ;RETURN LIST STATUS 4A30H+b JMP SECTRAN ;SECTOR TRANSLATE SUBROUTINE .fi .in 0 .sp 2 .sh Listing 6-2. BIOS Entry Points .pp Each jump address corresponds to a particular subroutine that performs the specific function, as outlined below. There are three major divisions in the jump table: the system reinitialization, which results from calls on BOOT and WBOOT; simple character I/O, performed by calls on CONST, CONIN, CONOUT, LIST, PUNCH, READER, and LISTST; and disk I/O, performed by calls on HOME, SELDSK, SETTRK, SETSEC, SETDMA, READ, WRITE, and SECTRAN. .pp All simple character I/O operations are assumed to be performed in ASCII, upper- and lower-case, with high-order (parity bit) set to zero. An end-of-file condition for an input device is given by an ASCII CTRL-Z (1AH). Peripheral devices are seen by CP/M as logical devices and are assigned to physical devices within the BIOS. .pp To operate, the BDOS needs only the CONST, CONIN, and CONOUT subroutines. LIST, PUNCH, and READER can be used by PIP, but not the BDOS. Further, the LISTST entry is currently used only by DESPOOL, the print spooling utility. Thus, the initial version of CBIOS can have empty subroutines for the remaining ASCII devices. .pp The following list describes the characteristics of each device. .sp 2 .in 5 .ti -2 o CONSOLE is the principal interactive console that communicates with the operator and it is accessed through CONST, CONIN, and CONOUT. Typically, the CONSOLE is a device such as a CRT or teletype. .sp .ti -2 o LIST is the principal listing device. If it exists on the user's system, it is usually a hard-copy device, such as a printer or teletype. .sp .ti -2 o PUNCH is the principal tape punching device. If it exists, it is normally a high-speed paper tape punch or teletype. .sp .ti -2 o READER is the principal tape reading device, such as a simple optical reader or teletype. .fi .in 0 .sp .pp A single peripheral can be assigned as the LIST, PUNCH, and READER device simultaneously. If no peripheral device is assigned as the LIST, PUNCH, or READER device, the CBIOS gives an appropriate error message so that the system does not hang if the device is accessed by PIP or some other user program. Alternately, the PUNCH and LIST routines can just simply return, and the READER routine can return with a 1AH (CTRL-Z) in register A to indicate immediate end-of-file. .pp For added flexibility, you can optionally implement the IOBYTE function, which allows reassignment of physical devices. The IOBYTE function creates a mapping of logical-to-physical devices that can be altered during CP/M processing, see the STAT command in Section 1.6.1. .pp The definition of the IOBYTE function corresponds to the Intel standard as follows: a single location in memory, currently location 0003H, is maintained, called IOBYTE, which defines the logical-to-physical device mapping that is in effect at a particular time. The mapping is performed by splitting the IOBYTE into four distinct fields of two bits each, called the CONSOLE, READER, PUNCH, and LIST fields, as shown in the following figure. .sp 2 .nf most significant least significant .sp IOBYTE AT 003H LIST PUNCH READER CONSOLE .sp bits 6,7 bits 4,5 bits 2,3 bits 0,1 .fi .sp 2 .sh Figure 6-1. IOBYTE Fields .sp 2 .pp The value in each field can be in the range 0-3, defining the assigned source or destination of each logical device. Table 6-4 gives the values that can be assigned to each field. .sp 2 .sh .nf Table 6-4. IOBYTE Field Values .sp Value Meaning .sp CONSOLE field (bits 0,1) .sp 0 console is assigned to the console printer device (TTY:) 1 console is assigned to the CRT device (CRT:) 2 batch mode: use the READER as the CONSOLE input, and the LIST device as the CONSOLE output (BAT:) 3 user-defined console device (UC1:) .sp .mb 4 .fm 1 .mt 4 .hm 1 READER field (bits 2,3) .sp 0 READER is the teletype device (TTY:) 1 READER is the high speed reader device (PTR:) 2 user-defined reader #1 (UR1:) 3 user-defined reader #2 (UR2:) .sp PUNCH field (bits 4,5) .sp 0 PUNCH is the teletype device (TTY:) 1 PUNCH is the high speed punch device (PTP:) 2 user-defined punch #1 (UP1:) 3 user-defined punch #2 (UP2:) .sp LIST field (bits 6,7) .sp 0 LIST is the teletype device (TTY:) 1 LIST is the CRT device (CRT:) 2 LIST is the line printer device (LPT:) 3 user-defined list device (UL1:) .fi .bp .pp The implementation of the IOBYTE is optional and effects only the organization of the CBIOS. No CP/M systems use the IOBYTE (although they tolerate the existence of the IOBYTE at location 0003H) except for PIP, which allows access to the physical devices, and STAT, which allows logical-physical assignments to be make or displayed. For more information see Section 1. In any case the IOBYTE implementation should be omitted until the basic CBIOS is fully implemented and tested; then you should add the IOBYTE to increase the facilities. .mb 6 .fm 2 .mt 5 .hm 2 .pp Disk I/O is always performed through a sequence of calls on the various disk access subroutines that set up the disk number to access, the track and sector on a particular disk, and the Direct Memory Access (DMA) address involved in the I/O operation. After all these parameters have been set up, a call is made to the READ or WRITE function to perform the actual I/O operation. .pp There is often a single call to SELDSK to select a disk drive, followed by a number of read or write operations to the selected disk before selecting another drive for subsequent operations. Similarly, there might be a single call to set the DMA address, followed by several calls that read or write from the selected DMA address before the DMA address is changed. The track and sector subroutines are always called before the READ or WRITE operations are performed. .pp The READ and WRITE routines should perform several retries (10 is standard) before reporting the error condition to the BDOS. If the error condition is returned to the BDOS, it reports the error to the user. The HOME subroutine might or might not actually perform the track 00 seek, depending upon controller characteristics; the important point is that track 00 has been selected for the next operation and is often treated in exactly the same manner as SETTRK with a parameter of 00. .pp The following table describes the exact responsibilities of each BIOS entry point subroutine. .sp 2 .sh Table 6-5. BIOS Entry Points .sp Entry Point Function .sp .ll 60 .in 15 .ti -9 BOOT The BOOT entry point gets control from the cold start loader and is responsible for basic system initialization, including sending a sign-on message, which can be omitted in the first version. If the IOBYTE function is implemented, it must be set at this point. The various system parameters that are set by the WBOOT entry point must be initialized, and control is transferred to the CCP at 3400+b for further processing. Note that register C must be set to zero to select drive A. .in 0 .bp .sh Table 6-5. (continued) .sp Entry Point Function .sp .in 15 .ti -9 WBOOT The WBOOT entry point gets control when a warm start occurs. A warm start is performed whenever a user program branches to location 0000H, or when the CPU is reset from the front panel. The CP/M system must be loaded from the first two tracks of drive A up to, but not including, the BIOS, or CBIOS, if the user has completed the patch. System parameters must be initialized as follows: .sp .in 32 .ti -17 location 0,1,2 Set to JMP WBOOT for warm starts (000H: JMP 4A03H+b) .sp .ti -17 location 3 Set initial value of IOBYTE, if implemented in the CBIOS .sp .ti -17 location 4 High nibble = current user no; low nibble = current drive .sp .ti -17 location 5,6,7 Set to JMP BDOS, which is the primary entry point to CP/M for transient programs. (0005H: JMP 3C06H+b) .sp .in 15 Refer to Section 6.9 for complete details of page zero use. Upon completion of the initialization, the WBOOT program must branch to the CCP at 3400H+b to restart the system. Upon entry to the CCP, register C is set to the drive to select after system initialization. The WBOOT routine should read location 4 in memory, verify that is a legal drive, and pass it to the CCP in register C. .sp .ti -9 CONST You should sample the status of the currently assigned console device and return 0FFH in register A if a character is ready to read and 00H in register A if no console characters are ready. .sp .ti -9 CONIN The next console character is read into register A, and the parity bit is set, high-order bit, to zero. If no console character is ready, wait until a character is typed before returning. .in 0 .bp .sh Table 6-5. (continued) .sp Entry Point Function .sp .in 15 .ti -9 CONOUT The character is sent from register C to the console output device. The character is in ASCII, with high-order parity bit set to zero. You might want to include a time-out on a line-feed or carriage return, if the console device requires some time interval at the end of the line (such as a TI Silent 700 terminal). You can filter out control characters that cause the console device to react in a strange way (CTRL-Z causes the Lear-Seigler terminal to clear the screen, for example). .sp .ti -9 LIST The character is sent from register C to the currently assigned listing device. The character is in ASCII with zero parity bit. .sp .ti -9 PUNCH The character is sent from register C to the currently assigned punch device. The character is in ASCII with zero parity. .sp .ti -9 READER The next character is read from the currently assigned reader device into register A with zero parity (high-order bit must be zero); an end-of-file condition is reported by returning an ASCII CTRL-Z(1AH). .sp .ti -9 HOME The disk head of the currently selected disk (initially disk A) is moved to the track 00 position. If the controller allows access to the track 0 flag from the drive, the head is stepped until the track 0 flag is detected. If the controller does not support this feature, the HOME call is translated into a call to SETTRK with a parameter of 0. .sp .ti -9 SELDSK The disk drive given by register C is selected for further operations, where register C contains 0 for drive A, 1 for drive B, and so on up to 15 for drive P (the standard CP/M distribution version supports four drives). On each disk select, SELDSK must return in HL the base address of a 16-byte area, called the Disk Parameter Header, described in Section 6.10. For standard floppy disk drives, the contents of the header and associated tables do not change; thus, the program segment included in the sample CBIOS performs this operation automatically. .in 0 .bp .sh Table 6-5. (continued) .sp Entry Point Function .sp .in 15 If there is an attempt to select a nonexistent drive, SELDSK returns HL=0000H as an error indicator. Although SELDSK must return the header address on each call, it is advisable to postpone the physical disk select operation until an I/O function (seek, read, or write) is actually performed, because disk selects often occur without utimately performing any disk I/O, and many controllers unload the head of the current disk before selecting the new drive. This causes an excessive amount of noise and disk wear. The least significant bit of register E is zero if this is the first occurrence of the drive select since the last cold or warm start. .sp .ti -9 SETTRK Register BC contains the track number for subsequent disk accesses on the currently selected drive. The sector number in BC is the same as the number returned from the SECTRAN entry point. You can choose to seek the selected track at this time or delay the seek until the next read or write actually occurs. Register BC can take on values in the range 0-76 corresponding to valid track numbers for standard floppy disk drives and 0-65535 for nonstandard disk subsystems. .sp .ti -9 SETSEC Register BC contains the sector number, 1 through 26, for subsequent disk accesses on the currently selected drive. The sector number in BC is the same as the number returned from the SECTRAN entry point. You can choose to send this information to the controller at this point or delay sector selection until a read or write operation occurs. .mb 4 .fm 1 .sp .ti -9 SETDMA Register BC contains the DMA (Disk Memory Access) address for subsequent read or write operations. For example, if B = 00H and C = 80H when SETDMA is called, all subsequent read operations read their data into 80H through 0FFH and all subsequent write operations get their data from 80H through 0FFH, until the next call to SETDMA occurs. The initial DMA address is assumed to be 80H. The controller need not actually support Direct Memory Access. If, for example, all data transfers are through I/O ports, the CBIOS that is constructed uses the 128-byte area starting at the selected DMA address for the memory buffer during the subsequent read or write operations. .in 0 .bp .sh Table 6-5. (continued) .sp Entry Point Function .sp .in 15 .ti -9 READ Assuming the drive has been selected, the track has been set, and the DMA address has been specified, the READ subroutine attempts to read one sector based upon these parameters and returns the following error codes in register A: .sp 0 no errors occurred .sp 1 nonrecoverable error condition occurred .sp Currently, CP/M responds only to a zero or nonzero value as the return code. That is, if the value in register A is 0, CP/M assumes that the disk operation was completed properly. IF an error occurs the CBIOS should attempt at least 10 retries to see if the error is recoverable. When an error is reported the BDOS prints the message BDOS ERR ONx: BAD SECTOR. The operator then has the option of pressing a carriage return to ignore the error, or CTRL-C to abort. .sp .ti -9 WRITE Data is written from the currently selected DMA address to the currently selected drive, track, and sector. For floppy disks, the data should be marked as nondeleted data to maintain compatibility with other CP/M systems. The error codes given in the READ command are returned in register A, with error recovery attempts as described above. .mb 6 .fm 2 .sp .ti -9 LISTST You return the ready status of the list device used by the DESPOOL program to improve console response during its operation. The value 00 is returned in A if the list device is not ready to accept a character and 0FFH if a character can be sent to the printer. A 00 value should be returned if LIST status is not implemented. .in 0 .bp .sh Table 6-5. (continued) .sp Entry Point Function .sp .in 15 .ti -9 SECTRAN Logical-to-physical sector translation is performed to improve the overall response of CP/M. Standard CP/M systems are shipped with a skew factor of 6, where six physical sectors are skipped between each logical read operation. This skew factor allows enough time between sectors for most programs to load their buffers without missing the next sector. In particular computer systems that use fast processors, memory, and disk subsystems, the skew factor might be changed to improve overall response. However, the user should maintain a single-density IBM-compatible version of CP/M for information transfer into and out of the computer system, using a skew factor of 6. .sp In general, SECTRAN receives a logical sector number relative to zero in BC and a translate table address in DE. The sector number is used as an index into the translate table, with the resulting physical sector number in HL. For standard systems, the table and indexing code is provided in the CBIOS and need not be changed. .in 0 .ll 65 .sp 2 .tc 6.7 A Sample BIOS .he CP/M Operating System Manual 6.7 A Sample BIOS .sh 6.7 A Sample BIOS .qs .pp The program shown in Appendix B can serve as a basis for your first BIOS. The simplest functions are assumed in this BIOS, so that you can enter it through a front panel, if absolutely necessary. You must alter and insert code into the subroutines for CONST, CONIN, CONOUT, READ, WRITE, and WAITIO subroutines. Storage is reserved for user-supplied code in these regions. The scratch area reserved in page zero (see Section 6.9) for the BIOS is used in this program, so that it could be implemented in ROM, if desired. .pp Once operational, this skeletal version can be enhanced to print the initial sign-on message and perform better error recovery. The subroutines for LIST, PUNCH, and READER can be filled out and the IOBYTE function can be implemented. .sp 2 .tc 6.8 A Sample Cold Start Loader .he CP/M Operating System Manual 6.8 A Sample Cold Start Loader .sh 6.8 A Sample Cold Start Loader .qs .pp The program shown in Appendix E can serve as a basis for a cold start loader. The disk read function must be supplied by the user, and the program must be loaded somehow starting at location 0000. Space is reserved for the patch code so that the total amount of storage required for the cold start loader is 128 bytes. .pp Eventually, you might want to get this loader onto the first disk sector (track 0, sector 1) and cause the controller to load it into memory automatically upon system start up. Alternatively, the cold start loader can be placed into ROM, and above the CP/M system. In this case, it is necessary to originate the program at a higher address and key in a jump instruction at system start up that branches to the loader. Subsequent warm starts do not require this key-in operation, because the entry point WBOOT gets control, thus bringing the system in from disk automatically. The skeletal cold start loader has minimal error recovery, which might be enhanced in later versions. .sp 2 .tc 6.9 Reserved Locations in Page Zero .he CP/M Operating System Manual 6.9 Reserved Locations in Page Zero .sh 6.9 Reserved Locations in Page Zero .qs .pp Main memory page zero, between locations 00H and 0FFH, contains several segments of code and data that are used during CP/M processing. The code and data areas are given in the following table. .sp 2 .sh Table 6-6. Reserved Locations in Page Zero .sp .nf Locations Contents .fi .sp .ll 60 .in 22 .ti -17 000H-0002H Contains a jump instruction to the warm start entry location 4A03H+b. This allows a simple programmed restart (JMP 0000H) or manual restart from the front panel. .sp .ti -17 0003H-0003H Contains the Intel standard IOBYTE is optionally included in the user's CBIOS (refer to Section 6.6). .sp .ti -17 0004H-0004H Current default drive number (0=A,...,15=P). .sp .ti -17 0005H-0007H Contains a jump instruction to the BDOS and serves two purposes: JMP 0005H provides the primary entry point to the BDOS, as described in Chapter 5, and LHLD 0006H brings the address field of the instruction to the HL register pair. This value is the lowest address in memory used by CP/M, assuming the CCP is being overlaid. The DDT program changes the address field to reflect the reduced memory size in debug mode. .sp .ti -17 0008H-0027H Interrupt locations 1 through 5 not used. .sp .ti -17 0030H-0037H Interrupt location 6 (not currently used) is reserved. .in 0 .bp .sh Table 6-6. (continued) .sp .nf Locations Contents .fi .sp .in 22 .ti -17 0038H-003AH Restart 7; contains a jump instruction into the DDT or SID program when running in debug mode for programmed breakpoints, but is not otherwise used by CP/M. .sp .ti -17 003BH-003FH Not currently used; reserved. .sp .ti -17 0040H-004FH A 16-byte area reserved for scratch by CBIOS, but is not used for any purpose in the distribution version of CP/M. .sp .ti -17 0050H-005BH Not currently used; reserved. .sp .ti -17 005CH-007CH Default File Control Block produced for a transient program by the CCP. .sp .ti -17 007DH-007FH Optional default random record position. .sp .ti -17 0080H-00FFH Default 128-byte disk buffer, also filled with the command line when a transient is loaded under the CCP. .in 0 .ll 65 .mb 4 .fm 1 .sp .pp This information is set up for normal operation under the CP/M system, but can be overwritten by a transient program if the BDOS facilities are not required by the transient. .pp If, for example, a particular program performs only simple I/O and must begin execution at location 0, it can first be loaded into the TPA, using normal CP/M facilities, with a small memory move program that gets control when loaded. The memory move program must get control from location 0100H, which is the assumed beginning of all transient programs. The move program can then proceed to the entire memory image down to location 0 and pass control to the starting address of the memory load. .pp If the BIOS is overwritten or if location 0, containing the warm start entry point, is overwritten, the operator must bring the CP/M system back into memory with a cold start sequence. .sp 2 .tc 6.10 Disk Parameter Tables .he CP/M Operating System Manual 6.10 Disk Parameter Tables .sh 6.10 Disk Parameter Tables .qs .pp Tables are included in the BIOS that describe the particular characteristics of the disk subsystem used with CP/M. These tables can be either hand-coded, as shown in the sample CBIOS in Appendix B, or automatically generated using the DISKDEF macro library, as shown in Appendix F. The purpose here is to describe the elements of these tables. .bp .pp In general, each disk drive has an associated (16-byte) disk parameter header that contains information about the disk drive and provides a scratch pad area for certain BDOS operations. The format of the disk parameter header for each drive is shown in Figure 6-2, where each element is a word (16-bit) value. .mb 6 .fm 2 .sp 3 .nf XLT 0000 0000 0000 DIRBUF DPB CSV ALV 16b 16b 16b 16b 16b 16b 16b 16b .fi .sp 2 .sh Figure 6-2. Disk Parameter Header Format .sp 2 .pp The meaning of each Disk Parameter Header (DPH) element is detailed in Table 6-7. .sp 2 .sh Table 6-7. Disk Parameter Headers .sp .nf Disk Parameter Meaning Header .fi .ll 60 .sp .in 20 .ti -14 XLT Address of the logical-to-physical translation vector, if used for this particular drive, or the value 0000H if no sector translation takes place (that is, the physical and logical sector numbers are the same). Disk drives with identical sector skew factors share the same translate tables. .sp .ti -14 0000 Scratch pad values for use within the BDOS, initial value is unimportant. .sp .ti -14 DIRBUF Address of a 128-byte scratch pad area for directory operations within BDOS. All DPHs address the same scratch pad area. .sp .ti -14 DPB Address of a disk parameter block for this drive. Drives with identical disk characteristics address the same disk parameter block. .sp .ti -14 CSV Address of a scratch pad area used for software check for changed disks. This address is different for each DPH. .sp .ti -14 ALV Address of a scratch pad area used by the BDOS to keep disk storage allocation information. This address is different for each DPH. .fi .in 0 .ll 65 .bp .pp Given n disk drives, the DPHs are arranged in a table whose first row of 16 bytes corresponds to drive 0, with the last row corresponding to drive n-1. In the following figure the lable DPBASE defines the base address of the DPH table. .sp 3 .nf DPBASE: .sp 00 XLT 00 0000 0000 0000 DIRBUF DBP 00 CSV 00 ALV 00 .sp 01 XLT 01 0000 0000 0000 DIRBUF DBP 01 CSV 01 ALV 01 . . . n-1 XLTn-1 0000 0000 0000 DIRBUF DBTn-1 CSVn-1 ALVn-1 .fi .sp 2 .sh Figure 6-3. Disk Parameter Header Table .sp 2 .pp A responsibility of the SELDSK subroutine is to return the base address of the DPH for the selected drive. The following sequence of operations returns the table address, with a 0000H returned if the selected drive does not exist. .sp 2 .nf .in 7 NDISKS EQU 4 ;NUMBER OF DISK DRIVES ..... SELDSK: ;SELECT DISK GIVEN BY BC LSI H,0000H ;ERROR CODE MOV A,C ;DRIVE OK? CPI NDISKS ;CY IF SO RNC ;RET IF ERROR ;NO ERROR, CONTINUE MOV L,C ;LOW(DISK) MOV H,B ;HIGH(DISK) DAD H ;*2 DAD H ;*4 DAD H ;*8 DAD H ;*16 LXI D,DPBASE;FIRST DPH DAD D ;DPH(DISK) RET .fi .in 0 .sp .pp The translation vectors, XLT 00 through XLTn-1, are located elsewhere in the BIOS, and simply correspond one-for-one with the logical sector numbers zero through the sector count 1. The Disk Parameter Block (DPB) for each drive is more complex. As shown in Figure 6-4, particular DPB, that is addressed by one or more DPHs, takes the general form: .sp 3 .nf SPT BSH BLM EXM DSM DRM AL0 AL1 CKS 0FF 16b 8b 8b 8b 16b 16b 8b 8b 16b 16b .fi .sp 2 .sh Figure 6-4. Disk Parameter Block Format .sp 3 where each is a byte or word value, as shown by the 8b or 16b indicator below the field. .pp The following field abbreviations are used in Figure 6-4: .sp 2 .in 5 .ti -2 o SPT is the total number of sectors per track. .sp .ti -2 o BSH is the data allocation block shift factor, determined by the data block allocation size. .sp .ti -2 o BLM is the data allocation block mask (2[BSH-1]). .sp .ti -2 o EXM is the extent mask, determined by the data block allocation size and the number of disk blocks. .sp .ti -2 o DSM determines the total storage capacity of the disk drive. .sp .ti -2 o DRM determines the total number of directory entries that can be stored on this drive. AL0, AL1 determine reserved directory blocks. .sp .ti -2 o CKS is the size of the directory check vector. .sp .ti -2 o 0FF is the number of reserved tracks at the beginning of the (logical) disk. .fi .in 0 .sp The values of BSH and BLM determine the data allocation size BLS, which is not an entry in the DPB. Given that the designer has selected a value for BLS, the values of BSH and BLM are shown Table 6-8. .sp 2 .sh Table 6-8. BSH and BLM Values .nf .sp .in 18 BLS BSH BLM .sp 1024 3 7 2048 4 15 4096 5 31 8192 6 63 16,384 7 127 .fi .in 0 .sp 2 where all values are in decimal. The value of EXM depends upon both the BLS and whether the DSM value is less than 256 or greater than 255, as shown in Table 6-9. .bp .sh Table 6-9. EXM Values .nf .sp BLS EXM values .sp .in 18 DSM<256 DSM>255 .sp 1024 0 N/A 2048 1 0 4096 3 1 8192 7 3 16,384 15 7 .fi .in 0 .sp .pp The value of DSM is the maximum data block number supported by this particular drive, measured in BLS units. The product (DSM+1) is the total number of bytes held by the drive and must be within the capacity of the physical disk, not counting the reserved operating system tracks. .pp The DRM entry is the one less than the total number of directory entries that can take on a 16-bit value. The values of AL0 and AL1, however, are determined by DRM. The values AL0 and AL1 can together be considered a string of 16-bits, as shown in Figure 6-5. .sp 3 .nf AL0 AL1 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 .fi .sp 2 .sh Figure 6-5. AL0 and AL1 .sp 2 .pp Position 00 corresponds to the high-order bit of the byte labeled AL0 and 15 corresponds to the low-order bit of the byte labeled AL1. Each bit position reserves a data block for number of directory entries, thus allowing a total of 16 data blocks to be assigned for directory entries (bits are assigned starting at 00 and filled to the right until position 15). Each directory entry occupies 32 bytes, resulting in the following tabulation: .sp 2 .sh Table 6-10. BLS Tabulation .sp .nf .in 18 BLS Directory Entries .sp 1024 32 times # bits 2048 64 times # bits 4096 128 times # bits 8192 256 times # bits 16,384 512 times # bits .fi .in 0 .bp .pp Thus, if DRM = 127 (128 directory entries) and BLS = 1024, there are 32 directory entries per block, requiring 4 reserved blocks. In this case, the 4 high-order bits of AL0 are set, resulting in the values AL0 = 0F0H and AL1 = 00H. .pp The CKS value is determined as follows: if the disk drive media is removable, then CKS = (DRM+1)/4, where DRM is the last directory entry number. If the media are fixed, then set CKS = 0 (no directory records are checked in this case). .pp Finally, the 0FF field determines the number of tracks that are skipped at the beginning of the physical disk. This value is automatically added whenever SETTRK is called and can be used as a mechanism for skipping reserved operating system tracks or for partitioning a large disk into smaller segmented sections. .pp To complete the discussion of the DPB, several DPHs can address the same DPB if their drive characteristics are identical. Further, the DPB can be dynamically changed when a new drive is addressed by simply changing the pointer in the DPH; because the BDOS copies the DPB values to a local area whenever the SELDSK function is invoked. .pp Returning back to DPH for a particular drive, the two address values CSV and ALV remain. Both addresses reference an area of uninitialized memory following the BIOS. The areas must be unique for each drive, and the size of each area is determined by the values in the DPB. .pp The size of the area addressed by CSV is CKS bytes, which is sufficient to hold the directory check information for this particular drive, If CKS = (DRM+1)/4, you must reserve (DRM+1)/4 bytes for directory check use. If CKS = 0, no storage is reserved. .pp The size of the area addressed by ALV is determined by the maximum number of data blocks allowed for this particular disk and is computed as (DSM/8)+1. .pp The CBIOS shown in Appendix B demonstrates an instance of these tables for standard 8-inch, single-density drives. It might be useful to examine this program and compare the tabular values with the definitions given above. .sp 2 .tc 6.11 The DISKDEF Macro Library .he CP/M Operating System Manual 6.11 The DISKDEF Macro Library .sh 6.11 The DISKDEF Macro Library .qs .pp A macro library called DISKDEF (shown in Appendix F), greatly simplifies the table construction process. You must have access to the MAC macro assembler, of course, to use the DISKDEF facility, while the macro library is included with all CP.M 2 distribution disks. .bp .pp A BIOS disk definition consists of the following sequence of macro statements: .sp .nf .in 7 MACLIB DISKDEF ..... DISKS n DISKDEF 0,... DISKDEF 1,... ..... DISKDEF n-1 ..... ENDEF .fi .in 0 .sp where the MACLIB statement loads the DISKDEF.LIB file, on the same disk as the BIOS, into MAC's internal tables. The DISKS macro call follows, which specifies the number of drives to be configured with the user's system, where n is an integer in the range 1 to 16. A series of DISKDEF macro calls then follow that define the characteristics of each logical disk, 0 through n-1, corresponding to logical drives A through P. The DISKS and DISKDEF macros generate the in-line fixed data tables described in the previous section and thus must be placed in a nonexecutable portion of the BIOS, typically directly following the BIOS jump vector. .pp The remaining portion of the BIOS is defined following the DISKDEF macros, with the ENDEF macro call immediately preceding the END statement. The ENDEF (End of Diskdef) macro generates the necessary uninitialized RAM areas that are located in memory above the BIOS. .pp The DISKDEF macro call takes the form: .sp .ti 8 DISKDEF dn,fsc,lsc,[skf],bls dks,dir,cks,ofs,[0] .sp where .sp .in 5 .ti -2 o dn is the logical disk number, 0 to n-1. .ti -2 o fsc is the first physical sector number (0 or 1). .ti -2 o lsc is the last sector number. .ti -2 o skf is the optional sector skew factor. .ti -2 o bls is the data allocation block size. .ti -2 o dks is the number of blocks on the disk. .ti -2 o dir is the number of directory entries. .ti -2 o cks is the number of checked directory entries. .ti -2 o ofs is the track offset to logical track 00. .ti -2 o [0] is an optional 1.4 compatibility flag. .fi .in 0 .sp .pp The value dn is the drive number being defined with this DISKDEF macro invocation. The fsc parameter accounts for differing sector numbering systems and is usually 0 to 1. The lsc is the last numbered sector on a track. When present, the skf parameter defines the sector skew factor, which is used to create a sector translation table according to the skew. .pp If the number of sectors is less than 256, a single-byte table is created, otherwise each translation table element occupies two bytes. No translation table is created if the skf parameter is omitted, or equal to 0. .pp The bls parameter specifies the number of bytes allocated to each data block, and takes on the values 1024, 2048, 4096, 8192, or 16384. Generally, performance increases with larger data block sizes because there are fewer directory references, and logically connected data records are physically close on the disk. Further, each directory entry addresses more data and the BIOS-resident RAM space is reduced. .pp The dks parameter specifies the total disk size in bls units. That is, if the bls = 2048 and dks = 1000, the total disk capacity is 2,048,000 bytes. If dks is greater than 255, the block size parameter bls must be greater than 1024. The value of dir is the total number of directory entries that might exceed 255, if desired. .pp The cks parameter determines the number of directory items to check on each directory scan and is used internally to detect changed disks during system operation, where an intervening cold or warm start has not occurred. When this situation is detected, CP/M automatically marks the disk Read-Only so that data is not subsequently destroyed. .pp As stated in the previous section, the value of cks = dir when the medium is easily changed, as is the case with a floppy disk subsystem. If the disk is permanently mounted, the value of cks is typically 0, because the probability of changing disks without a restart is low. .pp The ofs value determines the number of tracks to skip when this particular drive is addressed, which can be used to reserve additional operating system space or to simulate several logical drives on a single large capacity physical drive. Finally, the [0] parameter is included when file compatibility is required with versions of 1.4 that have been modified for higher density disks. This parameter ensures that only 16K is allocated for each directory record, as was the case for previous versions. Normally, this parameter is not included. .pp For convenience and economy of table space, the special form: .sp .ti 8 DISKDEF i,j .sp gives disk i the same characteristics as a previously defined drive j. A standard four-drive, single-density system, which is compatible with version 1.4, is defined using the following macro invocations: .sp .nf .in 7 DISKS 4 DISKDEF 0,1,26,6,1024,243,64,2 DISKDEF 1,0 DISKDEF 2,0 DISKDEF 3,0 .... ENDEF .fi .in 0 .sp with all disks having the same parameter values of 26 sectors per track, numbered 1 through 26, with 6 sectors skipped between each access, 1024 bytes per data block, 243 data blocks for a total of 243K-byte disk capacity, 64 checked directory entries, and two operating system tracks. .pp The DISKS macro generates n DPHs, starting at the DPH table address DPBASE generated by the macro. Each disk header block contains sixteen bytes, as described above, and correspond one-for-one to each of the defined drives. In the four-drive standard system, for example, the DISKS macro generates a table of the form: .sp .nf .in 5 DPBASE EQU$ DPE0: DW XLT0,0000H,0000H,0000H,DIRBUF,DPB0,CSV0,ALV0 DPE1: DW XLT0,0000H,0000H,0000H,DIRBUF,DPB0,CSV1,ALV1 DPE2: DW XLT0,0000H,0000H,0000H,DIRBUF,DPB0,CSV2,ALV2 DPE3: DW XLT0,0000H,0000H,0000H,DIRBUF,DPB0,CSV3,ALV3 .fi .in 0 .sp where the DPH labels are included for reference purposes to show the beginning table addresses for each drive 0 through 3. The values contained within the DPH are described in detail in the previous section. The check and allocation vector addresses are generated by the ENDEF macro in the ram area following the BIOS code and tables. .pp Note that if the skf (skew factor) parameter is omitted, or equal to 0, the translation table is omitted and a 0000H value is inserted in the XLT position of the DPH for the disk. In a subsequent call to perform the logical-to-physical translation, SECTRAN receives a translation table address of DE = 0000H and simply returns the original logical sector from BC in the HL register pair. .pp A translate table is constructed when the skf parameter is present, and the (nonzero) table address is placed into the corresponding DPHs. The following for example, is constructed when the standard skew factor skf = 6 is specified in the DISKDEF macro call: .sp .nf .in 8 XLT0: DB 1,7,13,19,25,5,11,17,23,3,9,15,21 DB 2,8,14,20,26,6,12,18,24,4,10,16,22 .fi .in 0 .pp Following the ENDEF macro call, a number of uninitialized data areas are defined. These data areas need not be a part of the BIOS that is loaded upon cold start, but must be available between the BIOS and the end of memory. The size of the uninitialized RAM area is determined by EQU statements generated by the ENDEF macro. For a standard four-drive system, the ENDEF macro might produce the following EQU statement: .bp .nf .in 8 4C72 = BEGDAT EQU $ (data areas) .sp 4DB0 = ENDDAT EQU $ .sp 013C = DATSIZ EQU $-BEGDAT .fi .in 0 .sp which indicates that uninitialized RAM begins at location 4C72H, ends at 4DB0H-1, and occupies 013CH bytes. You must ensure that these addresses are free for use after the system is loaded. .pp After modification, you can use the STAT program to check drive characteristics, because STAT uses the disk parameter block to decode the drive information. A STAT command of the form: .sp .ti 8 STAT d:DSK: .sp decodes the disk parameter block for drive d (d=A,...,P) and displays the following values: .sp 2 .nf .in 8 r: 128-byte record capacity k: kilobyte drive capacity d: 32-byte directory entries c: checked directory entries e: records/extent b: records/block s: sectors/track t: reserved tracks .fi .in 0 .sp .pp Three examples of DISKDEF macro invocations are shown below with corresponding STAT parameter values. The last example produces a full 8-megabyte system. .sp .nf .in 8 DISKDEF 0,1,58,,2048,256,128,128,2 r=4096, k=512, d=128, c=128, e=256, b=16, s=58, t=2 .sp DISKDEF 0,1,58,,2048,1024,300,0,2 r=16348, k=2048, d=300, c=0, e=128, b=16, s=58, t=2 .sp DISKDEF 0,1,58,,16348,512,128,128,2 r=65536, k=8192, d=128, c=128, e=1024, b=128, s=58, t=2 .fi .in 0 .sp 2 .tc 6.12 Sector Blocking and Deblocking .he CP/M Operating System Manual 6.12 Blocking and Deblocking .sh 6.12 Sector Blocking and Deblocking .qs .pp Upon each call to BIOS WRITE entry point, the CP/M BDOS includes information that allows effective sector blocking and deblocking where the host disk subsystem has a sector size that is a multiple of the basic 128-byte unit. The purpose here is to present a general-purpose algorithm that can be included within the BIOS and that uses the BDOS information to perform the operations automatically. .pp On each call to WRITE, the BDOS provides the following information in register C: .sp .nf .in 8 0 = (normal sector write) 1 = (write to directory sector) 2 = (write to the first sector of a new data block) .fi .in 0 .pp Condition 0 occurs whenever the next write operation is into a previously written area, such as a random mode record update; when the write is to other than the first sector of an unallocated block; or when the write is not into the directory area. Condition 1 occurs when a write into the directory area is performed. Condition 2 occurs when the first record (only) of a newly allocated data block is written. In most cases, application programs read or write multiple 128-byte sectors in sequence; thus, there is little overhead involved in either operation when blocking and deblocking records, because preread operations can be avoided when writing records. .pp Appendix G lists the blocking and deblocking algorithms in skeletal form; this file is included on your CP/M disk. Generally, the algorithms map all CP/M sector read operations onto the host disk through an intermediate buffer that is the size of the host disk sector. Throughout the program, values and variables that relate to the CP/M sector involved in a seek operation are prefixed by sek, while those related to the host disk system are prefixed by hst. The equate statements beginning on line 29 of Appendix G define the mapping between CP/M and the host system, and must be changed if other than the sample host system is involved. .pp The entry points BOOT and WBOOT must contain the initialization code starting on line 57, while the SELDSK entry point must be augmented by the code starting on line 65. Note that although the SELDSK entry point computes and returns the Disk Parameter Header address, it does not physically select the host disk at this point (it is selected later at READHST or WRITEHST). Further, SETTRK, SETTRK, and SETMA simply store the values, but do not take any other action at this point. SECTRAN performs a trivial function of returning the physical sector number. .pp The principal entry points are READ and WRITE, starting on lines 110 and 125, respectively. These subroutines take the place of your previous READ and WRITE operations. .pp The actual physical read or write takes place at either WRITEHST or READHST, where all values have been prepared: hstdsk is the host disk number, hsttrk is the host track number, and hstsec is the host sector number, which may require translation to physical sector number. You must insert code at this point that performs the full sector read or write into or out of the buffer at hstbuf of length hstsiz. All other mapping functions are performed by the algorithms. .pp This particular algorithm was tested using an 80-megabyte hard disk unit that was originally configured for 128-byte sectors, producing approximately 35 megabytes of formatted storage. When configured for 512-byte host sectors, usable storage increased to 57 megabytes, with a corresponding 400% improvement in overall response. In this situation, there is no apparent overhead involved in deblocking sectors, with the advantage that user programs still maintain 128-byte sectors. This is primarily because of the information provided by the BDOS, which eliminates the necessity for preread operations. .sp 2 .ce End of Section 6