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70 lines
2.0 KiB
70 lines
2.0 KiB
;
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;==================================================================================================
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; ENCODE 32-BIT VALUES TO A 5-BIT SHIFT-ENCODED VALUE
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;==================================================================================================
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;
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;
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; An encoded value (V) is defined as V = C * 2^X * 3^Y
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; where C is a prearranged constant, X is 0 or 1 and Y is 0-15
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; The encoded value is stored as 5 bits: YXXXX
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; At present, C=75 for baud rate encoding and C=3 for CPU OSC encoding
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;
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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; ENCODE
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;
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; Enter with:
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; DE:HL = dword value to be encoded
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; C = divisor (0 < C < 256)
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; encode divisor OSC_DIV = 3, or BAUD_DIV = 75
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;
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; Exit with:
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; C = encoded value
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; A = non-zero on error
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;
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ENCODE:
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; *** MAKE SURE INCOMING VALUE IS NOT ZERO???
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CALL ENCODE5 ; TEST DE:HL FOR ZERO
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JR Z,ENCODE4 ; IF ZERO, FAILURE RETURN
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; APPLY ENCODING DIVISOR
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CALL DIV32X8 ; DE:HL / C (REMAINDER IN A)
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OR A ; SET FLAGS TO TEST FOR ZERO
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RET NZ ; ERROR IF NOT EVENLY DIVISIBLE
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; APPLY DIV 3 IF POSSIBLE
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LD BC,ENCODE_TMP ; SAVE WORKING VALUE
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CALL ST32 ; ... IN TEMP
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LD C,3 ; ATTEMPT DIVIDE
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CALL DIV32X8 ; ... BY 3
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OR A ; SET FLAGS TO TEST FOR ZERO
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JR Z,ENCODE1 ; JUMP IF IT WORKED
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LD HL,ENCODE_TMP ; FAILED, RESTORE
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CALL LD32 ; ... PRIOR WORKING VALUE
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LD C,0 ; INIT RESULT IN C W/O DIV 3 FLAG
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JR ENCODE2
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ENCODE1:
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LD C,$10 ; INIT RESULT IN C W/ DIV 3 FLAG
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ENCODE2:
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; LOOP TO DETERMINE POWER OF 2
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LD B,32
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ENCODE3:
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SRL D
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RR E
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RR H
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RR L
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JR C,ENCODE5 ; DONE, C HAS RESULT
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INC C ; BUMP THE RESULT VALUE
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DJNZ ENCODE3
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ENCODE4:
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OR $FF ; SIGNAL ERROR
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RET ; AND DONE
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;
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ENCODE5:
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; TEST DE:HL FOR ZERO (SETS ZF, CLOBBERS A)
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LD A,H
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OR L
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OR D
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OR E
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RET ; RET W/ Z SET IF SUCCESSFUL
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;
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ENCODE_TMP .FILL 4,0 ; TEMP DWORD VALUE
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