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6502 BCD Tests
This program from 6502.org verifies 6502 Decimal Mode behavior.
;
; The following code comes from:
;
; http://www.6502.org/tutorials/decimal_mode.html
;
; and is built with:
;
; ca65 bcd.asm -I ../.. -l -o bcd.obj
; ld65 bcd.obj -o bcd.bin --target none
;
; An ASCII hex file which the OSI machine-language monitor can load
; is produced with the following commands:
;
; echo -n ".0200/" > bcd.txt
; hexdump -e '1/1 "%02X\n"' -v bcd.bin >> bcd.txt
; echo -n ".0200" >> bcd.txt
;
.include "sim.inc"
VALID_BCD_ONLY = 0
VALID_FLAGS_ONLY = 0
;
; Verify decimal mode behavior
;
; Returns:
; ERROR = 0 if the test passed
; ERROR = 1 if the test failed
;
; This routine requires 17 bytes of RAM -- 1 byte each for:
; AR, CF, DA, DNVZC, ERROR, HA, HNVZC, N1, N1H, N1L, N2, N2L, NF, VF, and ZF
; and 2 bytes for N2H
;
; Variables:
; N1 and N2 are the two numbers to be added or subtracted
; N1H, N1L, N2H, and N2L are the upper 4 bits and lower 4 bits of N1 and N2
; DA and DNVZC are the actual accumulator and flag results in decimal mode
; HA and HNVZC are the accumulator and flag results when N1 and N2 are
; added or subtracted using binary arithmetic
; AR, NF, VF, ZF, and CF are the predicted decimal mode accumulator and
; flag results, calculated using binary arithmetic
;
; This program takes approximately 1 minute at 1 MHz (a few seconds more on
; a 65C02 than a 6502 or 65816)
;
TEST: LDY #1 ; initialize Y (used to loop through carry flag values)
STY ERROR ; store 1 in ERROR until the test passes
LDA #0 ; initialize N1 and N2
STA N1
STA N2
LOOP1: LDA N2 ; N2L = N2 & $0F
SIM_MSG "Testing %A"
AND #$0F ; [1] see text
.if VALID_BCD_ONLY <> 0
CMP #$0A
BCS NEXT2
.endif
STA N2L
LDA N2 ; N2H = N2 & $F0
AND #$F0 ; [2] see text
.if VALID_BCD_ONLY <> 0
CMP #$A0
BCS NEXT2
.endif
STA N2H
ORA #$0F ; N2H+1 = (N2 & $F0) + $0F
STA N2H+1
LOOP2: LDA N1 ; N1L = N1 & $0F
;; SIM_MSG " with %A"
AND #$0F ; [3] see text
.if VALID_BCD_ONLY <> 0
CMP #$0A
BCS NEXT1
.endif
STA N1L
LDA N1 ; N1H = N1 & $F0
AND #$F0 ; [4] see text
.if VALID_BCD_ONLY <> 0
CMP #$A0
BCS NEXT1
.endif
STA N1H
JSR ADD
JSR A6502
JSR COMPARE
BNE ERR
JSR SUB
JSR S6502
JSR COMPARE
BNE ERR
NEXT1: INC N1 ; [5] see text
BNE LOOP2 ; loop through all 256 values of N1
NEXT2: INC N2 ; [6] see text
BNE LOOP1 ; loop through all 256 values of N2
DEY
BPL LOOP1 ; loop through both values of the carry flag
LDA #0 ; test passed, so store 0 in ERROR
STA ERROR
SIM_MSG "PASS"
BEQ DONE
ERR: SIM_MSG "FAIL"
DONE: SIM_HLT
; Calculate the actual decimal mode accumulator and flags, the accumulator
; and flag results when N1 is added to N2 using binary arithmetic, the
; predicted accumulator result, the predicted carry flag, and the predicted
; V flag
;
ADD: SED ; decimal mode
CPY #1 ; set carry if Y = 1, clear carry if Y = 0
LDA N1
ADC N2
STA DA ; actual accumulator result in decimal mode
PHP
PLA
STA DNVZC ; actual flags result in decimal mode
CLD ; binary mode
CPY #1 ; set carry if Y = 1, clear carry if Y = 0
LDA N1
ADC N2
STA HA ; accumulator result of N1+N2 using binary arithmetic
PHP
PLA
STA HNVZC ; flags result of N1+N2 using binary arithmetic
CPY #1
LDA N1L
ADC N2L
CMP #$0A
LDX #0
BCC A1
INX
ADC #5 ; add 6 (carry is set)
AND #$0F
SEC
A1: ORA N1H
;
; if N1L + N2L < $0A, then add N2 & $F0
; if N1L + N2L >= $0A, then add (N2 & $F0) + $0F + 1 (carry is set)
;
ADC N2H,X
PHP
BCS A2
CMP #$A0
BCC A3
A2: ADC #$5F ; add $60 (carry is set)
SEC
A3: STA AR ; predicted accumulator result
PHP
PLA
STA CF ; predicted carry result
PLA
;
; note that all 8 bits of the P register are stored in VF
;
STA VF ; predicted V flags
RTS
; Calculate the actual decimal mode accumulator and flags, and the
; accumulator and flag results when N2 is subtracted from N1 using binary
; arithmetic
;
SUB: SED ; decimal mode
CPY #1 ; set carry if Y = 1, clear carry if Y = 0
LDA N1
SBC N2
STA DA ; actual accumulator result in decimal mode
PHP
PLA
STA DNVZC ; actual flags result in decimal mode
CLD ; binary mode
CPY #1 ; set carry if Y = 1, clear carry if Y = 0
LDA N1
SBC N2
STA HA ; accumulator result of N1-N2 using binary arithmetic
PHP
PLA
STA HNVZC ; flags result of N1-N2 using binary arithmetic
RTS
; Calculate the predicted SBC accumulator result for the 6502 and 65816
;
SUB1: CPY #1 ; set carry if Y = 1, clear carry if Y = 0
LDA N1L
SBC N2L
LDX #0
BCS S11
INX
SBC #5 ; subtract 6 (carry is clear)
AND #$0F
CLC
S11: ORA N1H
;
; if N1L - N2L >= 0, then subtract N2 & $F0
; if N1L - N2L < 0, then subtract (N2 & $F0) + $0F + 1 (carry is clear)
;
SBC N2H,X
BCS S12
SBC #$5F ; subtract $60 (carry is clear)
S12: STA AR
RTS
; Calculate the predicted SBC accumulator result for the 6502 and 65C02
;
SUB2: CPY #1 ; set carry if Y = 1, clear carry if Y = 0
LDA N1L
SBC N2L
LDX #0
BCS S21
INX
AND #$0F
CLC
S21: ORA N1H
;
; if N1L - N2L >= 0, then subtract N2 & $F0
; if N1L - N2L < 0, then subtract (N2 & $F0) + $0F + 1 (carry is clear)
;
SBC N2H,X
BCS S22
SBC #$5F ; subtract $60 (carry is clear)
S22: CPX #0
BEQ S23
SBC #6
S23: STA AR ; predicted accumulator result
RTS
; Compare accumulator actual results to predicted results
;
; Return:
; Z flag = 1 (BEQ branch) if same
; Z flag = 0 (BNE branch) if different
;
COMPARE:LDA DA
CMP AR
BNE C1
.if VALID_FLAGS_ONLY = 0
LDA DNVZC ; [7] see text
EOR NF
AND #$80 ; mask off N flag
BNE C1
LDA DNVZC ; [8] see text
EOR VF
AND #$40 ; mask off V flag
BNE C1 ; [9] see text
LDA DNVZC
EOR ZF ; mask off Z flag
AND #2
BNE C1 ; [10] see text
.endif
LDA DNVZC
EOR CF
AND #1 ; mask off C flag
C1: RTS
; These routines store the predicted values for ADC and SBC for the 6502,
; 65C02, and 65816 in AR, CF, NF, VF, and ZF
A6502: LDA VF
;
; since all 8 bits of the P register were stored in VF, bit 7 of VF contains
; the N flag for NF
;
STA NF
LDA HNVZC
STA ZF
RTS
S6502: JSR SUB1
LDA HNVZC
STA NF
STA VF
STA ZF
STA CF
RTS
A65C02: LDA AR
PHP
PLA
STA NF
STA ZF
RTS
S65C02: JSR SUB2
LDA AR
PHP
PLA
STA NF
STA ZF
LDA HNVZC
STA VF
STA CF
RTS
A65816: LDA AR
PHP
PLA
STA NF
STA ZF
RTS
S65816: JSR SUB1
LDA AR
PHP
PLA
STA NF
STA ZF
LDA HNVZC
STA VF
STA CF
RTS
.data
AR: .byte 0
CF: .byte 0 ; predicted carry result
DA: .byte 0
DNVZC: .byte 0
ERROR: .byte 0
HA: .byte 0
HNVZC: .byte 0
N1: .byte 0
N1H: .byte 0
N1L: .byte 0
N2: .byte 0
N2L: .byte 0
NF: .byte 0
VF: .byte 0
ZF: .byte 0
N2H: .word 0