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81 lines
1.5 KiB
Plaintext
81 lines
1.5 KiB
Plaintext
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#if not(ARCH_6502)
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#warn bcd_6502 module should be only used on 6502-compatible targets
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#endif
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// subtracts __reg+3 from __reg+2 in a decimal way respecting carry
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// returns the result in A and C
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asm byte __sbc_decimal() {
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LDA #$99
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ADC #0
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SEC
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SBC __reg+3
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STA __reg+3
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CLC
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? JMP __adc_decimal
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}
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// subtracts __reg+3 from __reg+2 in a decimal way ignoring carry
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// returns the result in A and C
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asm byte __sub_decimal() {
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LDA #$9A
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SEC
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SBC __reg+3
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STA __reg+3
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CLC
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? JMP __adc_decimal
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}
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// adds __reg+2 and __reg+3 in a decimal way
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// returns the result in A and C
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asm byte __adc_decimal() {
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LDA __reg+2
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TAX
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AND #$F
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STA __reg+2
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LDA __reg+3
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AND #$F
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ADC __reg+2
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STX __reg+2
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CMP #$A
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BCC __adc_decimal_lo_no_carry
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// not needed: SEC
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ADC #5
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AND #$F
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TAX // X contains the sum of low digits
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LDA #$10
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CLC
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ADC __reg+2
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STA __reg+2 // halfcarry pushed into R2
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? JMP __adc_decimal_after_hi
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__adc_decimal_lo_no_carry:
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TAX
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__adc_decimal_after_hi:
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CLC
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LDA __reg+2
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AND #$F0
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ADC __reg+3
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AND #$F0
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BCS __adc_decimal_hi_carry
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STA __reg+2 // R2 contains the sum of high digits
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TXA
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CLC
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ADC __reg+2
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CMP #$A0
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BCC __adc_decimal_hi_no_carry
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// ; not needed: SEC
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SBC #$A0
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SEC
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__adc_decimal_hi_no_carry:
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RTS
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__adc_decimal_hi_carry:
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// ; not needed: SEC
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SBC #$A0
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STA __reg+2 // R2 contains the sum of high digits
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TXA
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CLC
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ADC __reg+2
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SEC
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RTS
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} |