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2 Features of B/P Bios
B/P BIOS is designed to be completely compatible with the CP/M 2.2 standards for a Basic IO System, as well as to provide many extensions needed for banked memory which is becoming so prevalent with newer systems and processors. Additionally, strict coding standards used in the various modules forming the BIOS ease interface problems with applications programs and provide a more robust framework for future development. The extensions added to the basic CP/M 2.2 foundation include many elements from Digital Research's CP/M 3 (aka CP/M Plus), but in a more logically consistent manner. Also included in banked versions are provisions for managing up to 8 MB of extended memory for banked applications, RAM Drives and potentially multitasking in future versions. To provide insight into the methodology used, let us now examine some of the features in a generic B/P Bios.
2.1 Character IO
As defined by Digital Research in their CP/M 2.2 standards, character IO consisted of logical devices referred to as TTY, CRT, UC1, CON, etc. B/P Bios extends and generalizes these interfaces using the IOBYTE to define four physical devices called COM1, COM2, PIO and NUL. The first two, COM1 and COM2, are serial ports; PIO is a Parallel port, while NUL is a "bit-bucket" which can be replaced by a customized driver, or used in lieu of an actual device. Digital Research provided only a limited interface capability to the character devices in CP/M 2.2, consisting of a Console (CON), an auxiliary Input and Output (RDR/PUN), and a Printer (LST). The ability to sense Input and Output Status with these devices was extremely limited and was enhanced in CP/M 3. These enhanced capabilities are completely incorporated into B/P Bios with the addition of strict register usage so that only relevant registers may be altered in the respective routines. By manipulating the IOBYTE, any of the four physical devices may be used in the three logical devices of CONsole, AUXiliary, and Printer (LST).
Also featured in B/P Bios are modifications of CP/M 3 functions to initialize (or re-initialize) all devices and parameters, and return the address of a table which contains names and parameters of the defined character devices. While not totally compatible with CP/M 3 equivalents, these functions are consistent with the spirit and functionality needed with this advanced system. Included in the device table are; flags defining whether the device is capable of Input, Output or Both, Data rates for serial devices (Maximum and Set), Serial data format where applicable, and Handshaking method (CTS/RTS, XON/XOFF or None), as well as Input and Output Data masks for stripping unneeded bits from characters during IO.
2.2 Mass Storage IO
All versions of Digital Research's CP/M BIOSes define only a generic Disk driver with implementations of Floppy, Hard, RAM and Tape drives left to the user or developer. In B/P Bios, we went several steps further to ease many problems. First, we retained all standard CP/M 2.2 functions and parameters, added CP/M 3 features for returning the Disk Parameter Header (DPH) table address, and flushing of the software deblocking code segment, and added a new vector to the BIOS jump table to provide a standard method of directly addressing low-level device functions. Several standard low-level Floppy Disk functions are supported and used by the standard utilities, including a function to return the type of Disk Controller in use which permits a single support utility to adapt to a wide variety of hardware platforms. In a like manner, low-level functions are provided for SCSI/SASI Hard Disk drives, and provisions for RAM Disk drives in the event special hardware is implemented. The methods used to implement these access mechanisms may be logically extended to handle Tape Drives or Network Interfaces.
2.3 Clock Support for Time and Date
Many Hardware vendors have added provisions for Time and Date as non-standard extensions to CP/M 2.2 BIOSes, and more have incorporated such support into CP/M 3 BIOSes. We opted to define the CP/M 3 clock vector as a ZSDOS-standard clock building on our previous Operating System work. This entry point into the Bios completely complies with our ZSDOS standards and can completely replace the separate clock driver when used with ZSDOS. For systems capable of returning tenths-of-seconds, such as the YASBEC and SB-180, the standard has been enhanced to support this capability as well.
2.4 Banked Memory Support
While Digital Research added banked memory support to their CP/M 3, it was in a manner incompatible with Bios interface standards defined for earlier CP/M standards. The method used in B/P Bios is compliant with CP/M 2.2 in direct accessing of Bios functions with only one minor exception when using the Banked ZSDOS2, and contains many of the CP/M 3 extensions added for banked memory support, with some being modified to be consistent with standards adopted for Z-System software. The exception to CP/M 2.2 accesses occurs when the Operating System can access certain buffers in the System Memory Bank. With ZSDOS 2, Allocation Bit Buffers (ALV), Check Buffers (CSV), and the Disk Host Buffer are all contained in the System Bank and not directly accessible from Transient Programs. To compensate for this, we have added a command to ZSDOS 2 to return the free space on disks (the most common reason for accessing these buffers) and tailored several utilities to adapt to banked and non-banked systems.
In addition to the primitives initiated by Digital Research, we added functions to directly access Words and Bytes in extended banks of memory, Directly accessing software routines contained in alternate memory banks, and properly managing the system when errors occur. These features make B/P Bios much more robust and resilient than other products. These features are implemented by methods transparent to the system utilities so that the same functions are available in both banked and non-banked versions.
2.5 Other Features
B/P Bios contains a standardized identification method which may be used to determine the hardware on which the software is operating. This allows applications to "adapt" to the environment in a manner similar to that used in the rest of the Z-System community. It also minimizes system "crashes" by executing programs which assume certain hardware features which may be detrimental if executed on other systems. The effects of identification of physical system parameters is most readily noticed by virtue of a single suite of support programs performing low-level functions such as formatting and diagnostics which function across widely differing hardware platforms. Portability on this scale can rarely be seen in other computer systems.
The ZCPR 3.4 Environment with extensions is mandatory in a B/P Bios system. Beginning with the addition of System Segment address and size information for CPR, DOS and BIOS which were added in the ZCPR 3.4 Environment, B/P Bios also adds a Resident User Space which may be used to locate unique routines for custom applications in a manner similar to, but more consistent than NZ-COM. An Environment Version number of 90H identifies the Z3 Environment as being compliant with B/P definitions.
In Banked systems, application programs may also be placed in alternate memory banks using location and sizing information contained at standard positions within the Bios Header Structure. This feature permits significantly greater functionality without sacrificing precious Transient Program Area. While the scheme employed in the initial distribution is subject to minor adjustments as the banked ZSDOS2 becomes more firmly developed, experimentation and suggestions into this realm are encouraged.