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285 lines
7.0 KiB
C
285 lines
7.0 KiB
C
/*
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* mos6502.arith.c
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*
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* We define here the logic for arithmetic instructions for the MOS 6502
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* processor.
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*/
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#include "mos6502.h"
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#include "mos6502.enums.h"
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/*
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* The adc instruction will add a number to the accumulator, "with
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* carry". If the carry bit is set, we will add 1 to the accumulator as
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* an after-effect.
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*/
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DEFINE_INST(adc)
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{
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if (cpu->P & MOS_DECIMAL) {
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mos6502_handle_adc_dec(cpu, oper);
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return;
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}
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MOS_CARRY_BIT();
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vm_8bit result = cpu->A + oper + carry;
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MOS_CHECK_NVZ(cpu->A, result);
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// Carry has different meanings in different contexts... in ADC,
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// carry is set if the result requires a ninth bit (the carry bit!)
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// to be set.
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cpu->P &= ~MOS_CARRY;
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if ((cpu->A + oper + carry) > 0xff) {
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cpu->P |= MOS_CARRY;
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}
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cpu->A = result;
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}
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/*
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* Add a number to the accumulator using Binary-Coded Decimal, or BCD.
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* Some things still work the same--for instance, if carry is high, we
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* will still add one to A+M. The V flag doesn't make any sense in BCD,
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* and apparently ADC's effects on V in decimal mode is "undocumented"?
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* The N flag is also a bit odd, and the general wisdom seems to be that
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* you should use multiple bytes if you want to represent a negative,
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* and just use the sign bit on the MSB.
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*/
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DEFINE_INST(adc_dec)
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{
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MOS_CARRY_BIT();
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// Determine the most and least siginificant digits of A and oper.
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int a_msd = cpu->A >> 4,
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a_lsd = cpu->A & 0xf,
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o_msd = oper >> 4,
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o_lsd = oper & 0xf;
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// If any of these are greater than 9, then they are invalid BCD
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// values, and we give up.
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if (a_msd > 9 || a_lsd > 9 || o_msd > 9 || o_lsd > 9) {
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return;
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}
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// Sum is built using the decimal senses of the msd/lsd variables;
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// carry is also a factor.
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int sum =
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((a_msd * 10) + a_lsd) +
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((o_msd * 10) + o_lsd) + carry;
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// But ultimately, one byte can only hold $00 - $99
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int modsum = sum % 100;
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// And the final result has to be ported back into a "hexadecimal"
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// number; you see, BCD values are not just literally decimal, they
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// are decimal in hexadecimal form.
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vm_8bit result = ((modsum / 10) << 4) | (modsum % 10);
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// As you can see, decimal comports a different meaning for the
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// carry bit than its binary version
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cpu->P &= ~MOS_CARRY;
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if (sum > 100) {
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cpu->P |= MOS_CARRY;
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}
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MOS_CHECK_Z(result);
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cpu->A = result;
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}
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/*
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* The cmp instruction will consider the difference of the accumulator
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* minus the operand. It will then set the zero, negative, or carry bits
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* based upon that difference. _The accumulator is neither modified nor
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* harmed by this operation._ (We have trained experts on the set to
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* monitor the health of the accumulator, whom we've named George.)
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*/
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DEFINE_INST(cmp)
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{
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MOS_CHECK_NZ(cpu->A - oper);
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// With CMP, carry is set if the difference between A and oper is
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// not zero.
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cpu->P &= ~MOS_CARRY;
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if (cpu->A >= oper) {
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cpu->P |= MOS_CARRY;
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}
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}
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/*
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* This instruction is functionally identical to CMP, with the exception
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* that it considers the X register rather than the accumulator.
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*/
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DEFINE_INST(cpx)
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{
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MOS_CHECK_NZ(cpu->X - oper);
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cpu->P &= ~MOS_CARRY;
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if (cpu->X >= oper) {
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cpu->P |= MOS_CARRY;
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}
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}
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/*
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* Again, this is a variant of the CMP instruction, except that it works
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* with the Y register.
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*/
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DEFINE_INST(cpy)
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{
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MOS_CHECK_NZ(cpu->Y - oper);
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cpu->P &= ~MOS_CARRY;
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if (cpu->Y >= oper) {
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cpu->P |= MOS_CARRY;
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}
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}
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/*
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* Here we will decrement the value at the effective address in memory
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* by 1.
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*/
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DEFINE_INST(dec)
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{
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if (cpu->eff_addr) {
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MOS_CHECK_NZ(oper - 1);
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mos6502_set(cpu, cpu->eff_addr, oper - 1);
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return;
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}
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// If we get here, then this is ACC mode, and we should work off
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// that.
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MOS_CHECK_NZ(cpu->A - 1);
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cpu->A--;
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}
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/*
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* In contrast, this does directly decrement the X register.
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*/
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DEFINE_INST(dex)
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{
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MOS_CHECK_NZ(cpu->X - 1);
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cpu->X--;
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}
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/*
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* And, again, here we decrement the Y register.
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*/
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DEFINE_INST(dey)
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{
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MOS_CHECK_NZ(cpu->Y - 1);
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cpu->Y--;
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}
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/*
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* The INC instruction is basically the same as the DEC one.
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*/
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DEFINE_INST(inc)
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{
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if (cpu->eff_addr) {
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MOS_CHECK_NZ(oper + 1);
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mos6502_set(cpu, cpu->eff_addr, oper + 1);
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return;
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}
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MOS_CHECK_NZ(cpu->A + 1);
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cpu->A++;
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}
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/*
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* See DEX.
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*/
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DEFINE_INST(inx)
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{
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MOS_CHECK_NZ(cpu->X + 1);
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cpu->X++;
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}
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/*
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* See DEY.
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*/
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DEFINE_INST(iny)
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{
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MOS_CHECK_NZ(cpu->Y + 1);
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cpu->Y++;
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}
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/*
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* This instruction will subtract the operand from the accumulator,
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* again, "with carry". In this context, that means that if the carry
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* bit is set, then we will subtract 1 again from the A register after
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* the operand is subtracted. The result is stored in the accumulator.
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*/
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DEFINE_INST(sbc)
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{
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// Jump into the binary coded decimal world with us! It's... funky!
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if (cpu->P & MOS_DECIMAL) {
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mos6502_handle_sbc_dec(cpu, oper);
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return;
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}
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MOS_CARRY_BIT();
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vm_8bit result = cpu->A - oper - (carry ? 0 : 1);
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MOS_CHECK_NVZ(cpu->A, result);
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// Carry is handled slightly differently in SBC; it's set if the
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// value is non-negative, and unset if negative. (It's essentially a
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// mirror of the N flag in that sense.)
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cpu->P |= MOS_CARRY;
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if (result >= 0x80) {
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cpu->P &= ~MOS_CARRY;
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}
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cpu->A = result;
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}
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/*
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* Pretty similar to the code we're doing in adc_dec; we are here
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* performing a subtraction in binary coded decimal.
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*
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* Note: a lot of this code is lifted from adc_dec; it's probably the
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* only other time we will need to use this specific code, so I'm doing
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* the Martin Fowler thing and waiting for a third occasion to arise
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* before refactoring this into its own function.
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*/
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DEFINE_INST(sbc_dec)
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{
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MOS_CARRY_BIT();
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// Determine the most and least siginificant digits of A and oper.
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int a_msd = cpu->A >> 4,
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a_lsd = cpu->A & 0xf,
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o_msd = oper >> 4,
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o_lsd = oper & 0xf;
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// If any of these are greater than 9, then they are invalid BCD
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// values, and we give up.
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if (a_msd > 9 || a_lsd > 9 || o_msd > 9 || o_lsd > 9) {
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return;
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}
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// Sum is built using the decimal senses of the msd/lsd variables;
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// carry is also a factor.
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int diff =
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((a_msd * 10) + a_lsd) -
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((o_msd * 10) + o_lsd) - (carry ? 0 : 1);
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// Force C to high to begin with
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cpu->P |= MOS_CARRY;
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// If diff is negative, we need to "overflow" it back to a
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// positive number by adding 100. We also need to unset the C flag.
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if (diff < 0) {
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diff += 100;
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cpu->P &= ~MOS_CARRY;
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}
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// And the final result has to be ported back into a "hexadecimal"
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// number; you see, BCD values are not just literally decimal, they
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// are decimal in hexadecimal form.
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vm_8bit result = ((diff / 10) << 4) | (diff % 10);
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MOS_CHECK_Z(result);
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cpu->A = result;
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}
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