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Rewrite status checks with macros

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This also forces the result of operations to be considered purely as
8-bit results in _some_ cases, but not in others. We were missing some
times when an overflow is the expected behavior; because SET_RESULT()
and mos6502_modify_status() use ints, this overflow was not properly
observed.
This commit is contained in:
Peter Evans 2018-02-28 21:16:39 -06:00
parent 4baec33e24
commit 49199eac84
4 changed files with 110 additions and 76 deletions
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31
include/mos6502.h
View File

@ -53,6 +53,37 @@
#define DECL_INST(x) \
extern void mos6502_handle_##x (mos6502 *, vm_8bit)
#define MOS_CHECK_Z(result) \
cpu->P &= ~MOS_ZERO; \
if ((vm_8bit)(result) == 0) cpu->P |= MOS_ZERO
#define MOS_CHECK_N(result) \
cpu->P &= ~MOS_NEGATIVE; \
if ((vm_8bit)(result) & 0x80) cpu->P |= MOS_NEGATIVE
#define MOS_CHECK_V(orig, result) \
cpu->P &= ~MOS_OVERFLOW; \
do { \
vm_8bit r = result; \
vm_8bit o = orig; \
if ((o & 0x80) ^ (r & 0x80)) { \
cpu->P |= MOS_OVERFLOW; \
} \
} while (0)
#define MOS_CHECK_NV(orig, result) \
MOS_CHECK_N(result); \
MOS_CHECK_V(orig, result)
#define MOS_CHECK_NZ(result) \
MOS_CHECK_N(result); \
MOS_CHECK_Z(result)
#define MOS_CHECK_NVZ(orig, result) \
MOS_CHECK_N(result); \
MOS_CHECK_V(orig, result); \
MOS_CHECK_Z(result)
typedef struct {
/*
* There are two different segment pointers for reading and writing,

57
src/mos6502.arith.c
View File

@ -21,11 +21,19 @@ DEFINE_INST(adc)
}
MOS_CARRY_BIT();
SET_RESULT(cpu->A + oper + carry);
mos6502_modify_status(cpu, MOS_NVZC, cpu->A, result);
vm_8bit result = cpu->A + oper + carry;
MOS_CHECK_NVZ(cpu->A, result);
cpu->A = result & 0xff;
// Carry has different meanings in different contexts... in ADC,
// carry is set if the result requires a ninth bit (the carry bit!)
// to be set.
cpu->P &= ~MOS_CARRY;
if ((cpu->A + oper + carry) > 0xff) {
cpu->P |= MOS_CARRY;
}
cpu->A = result;
}
/*
@ -65,17 +73,16 @@ DEFINE_INST(adc_dec)
// And the final result has to be ported back into a "hexadecimal"
// number; you see, BCD values are not just literally decimal, they
// are decimal in hexadecimal form.
SET_RESULT(((modsum / 10) << 4) | (modsum % 10));
vm_8bit result = ((modsum / 10) << 4) | (modsum % 10);
// Because the rules for carry is a bit different, we'll handle it
// here.
// As you can see, decimal comports a different meaning for the
// carry bit than its binary version
cpu->P &= ~MOS_CARRY;
if (sum > 100) {
cpu->P |= MOS_CARRY;
}
// We'll check zero, but not negative or overflow.
mos6502_modify_status(cpu, MOS_ZERO, cpu->A, sum);
MOS_CHECK_Z(result);
cpu->A = result;
}
@ -89,10 +96,10 @@ DEFINE_INST(adc_dec)
*/
DEFINE_INST(cmp)
{
mos6502_modify_status(cpu, MOS_NZ, cpu->A, cpu->A - oper);
MOS_CHECK_NZ(cpu->A - oper);
// Carry works slightly different with the cmp-style instructions;
// it's set if A >= oper.
// With CMP, carry is set if the difference between A and oper is
// not zero.
cpu->P &= ~MOS_CARRY;
if (cpu->A >= oper) {
cpu->P |= MOS_CARRY;
@ -105,7 +112,7 @@ DEFINE_INST(cmp)
*/
DEFINE_INST(cpx)
{
mos6502_modify_status(cpu, MOS_NZ, cpu->X, cpu->X - oper);
MOS_CHECK_NZ(cpu->X - oper);
cpu->P &= ~MOS_CARRY;
if (cpu->X >= oper) {
@ -119,7 +126,7 @@ DEFINE_INST(cpx)
*/
DEFINE_INST(cpy)
{
mos6502_modify_status(cpu, MOS_NZ, cpu->Y, cpu->Y - oper);
MOS_CHECK_NZ(cpu->Y - oper);
cpu->P &= ~MOS_CARRY;
if (cpu->Y >= oper) {
@ -134,7 +141,7 @@ DEFINE_INST(cpy)
DEFINE_INST(dec)
{
if (cpu->eff_addr) {
mos6502_modify_status(cpu, MOS_NZ, oper, oper - 1);
MOS_CHECK_NZ(oper - 1);
mos6502_set(cpu, cpu->eff_addr, oper - 1);
return;
}
@ -149,7 +156,7 @@ DEFINE_INST(dec)
*/
DEFINE_INST(dex)
{
mos6502_modify_status(cpu, MOS_NZ, cpu->X, cpu->X - 1);
MOS_CHECK_NZ(cpu->X - 1);
cpu->X--;
}
@ -158,7 +165,7 @@ DEFINE_INST(dex)
*/
DEFINE_INST(dey)
{
mos6502_modify_status(cpu, MOS_NZ, cpu->Y, cpu->Y - 1);
MOS_CHECK_NZ(cpu->Y - 1);
cpu->Y--;
}
@ -168,7 +175,7 @@ DEFINE_INST(dey)
DEFINE_INST(inc)
{
if (cpu->eff_addr) {
mos6502_modify_status(cpu, MOS_NZ, oper, oper + 1);
MOS_CHECK_NZ(oper + 1);
mos6502_set(cpu, cpu->eff_addr, oper + 1);
return;
}
@ -181,7 +188,7 @@ DEFINE_INST(inc)
*/
DEFINE_INST(inx)
{
mos6502_modify_status(cpu, MOS_NZ, cpu->X, cpu->X + 1);
MOS_CHECK_NZ(cpu->X + 1);
cpu->X++;
}
@ -190,7 +197,7 @@ DEFINE_INST(inx)
*/
DEFINE_INST(iny)
{
mos6502_modify_status(cpu, MOS_NZ, cpu->Y, cpu->Y + 1);
MOS_CHECK_NZ(cpu->Y + 1);
cpu->Y++;
}
@ -209,7 +216,9 @@ DEFINE_INST(sbc)
}
MOS_CARRY_BIT();
SET_RESULT(cpu->A - oper - (carry ? 0 : 1));
vm_8bit result = cpu->A - oper - (carry ? 0 : 1);
MOS_CHECK_NVZ(cpu->A, result);
// Carry is handled slightly differently in SBC; it's set if the
// value is non-negative, and unset if negative. (It's essentially a
@ -219,8 +228,7 @@ DEFINE_INST(sbc)
cpu->P &= ~MOS_CARRY;
}
mos6502_modify_status(cpu, MOS_NVZ, cpu->A, result);
cpu->A = result & 0xff;
cpu->A = result;
}
/*
@ -267,9 +275,8 @@ DEFINE_INST(sbc_dec)
// And the final result has to be ported back into a "hexadecimal"
// number; you see, BCD values are not just literally decimal, they
// are decimal in hexadecimal form.
SET_RESULT(((diff / 10) << 4) | (diff % 10));
mos6502_modify_status(cpu, MOS_ZERO, cpu->A, result);
vm_8bit result = ((diff / 10) << 4) | (diff % 10);
MOS_CHECK_Z(result);
cpu->A = result;
}

68
src/mos6502.bits.c
View File

@ -14,10 +14,8 @@
*/
DEFINE_INST(and)
{
SET_RESULT(cpu->A & oper);
mos6502_modify_status(cpu, MOS_NZ, cpu->A, result);
cpu->A = result & 0xff;
MOS_CHECK_NZ(cpu->A & oper);
cpu->A &= oper;
}
/*
@ -33,14 +31,18 @@ DEFINE_INST(and)
*/
DEFINE_INST(asl)
{
SET_RESULT(oper << 1);
vm_8bit result = oper << 1;
mos6502_modify_status(cpu, MOS_NZC, oper, result);
MOS_CHECK_NZ(result);
cpu->P &= ~MOS_CARRY;
if ((oper << 1) > 0xff) {
cpu->P |= MOS_CARRY;
}
if (cpu->eff_addr) {
mos6502_set(cpu, cpu->eff_addr, result & 0xff);
mos6502_set(cpu, cpu->eff_addr, result);
} else {
cpu->A = result & 0xff;
cpu->A = result;
}
}
@ -96,10 +98,8 @@ DEFINE_INST(bim)
*/
DEFINE_INST(eor)
{
SET_RESULT(cpu->A ^ oper);
mos6502_modify_status(cpu, MOS_NZ, cpu->A, result);
cpu->A = result & 0xff;
MOS_CHECK_NZ(cpu->A ^ oper);
cpu->A ^= oper;
}
/*
@ -111,27 +111,25 @@ DEFINE_INST(eor)
*/
DEFINE_INST(lsr)
{
SET_RESULT(oper >> 1);
vm_8bit result = oper >> 1;
// Set ZERO if it should apply; also inspect the NEGATIVE flag, but
// note that doing so will have the effect of _always_ unsetting the
// N flag, because literally any byte value shifted right will
// immediately lose the sign bit and be non-negative.
mos6502_modify_status(cpu, MOS_NZ, oper, result);
// The N flag is ALWAYS cleared in LSR, because a zero is always
// entered as bit 7
cpu->P &= ~MOS_NEGATIVE;
// However, we handle carry a bit differently here. The carry bit
// should be 1 if oper & 0x1; that is, when we shift right, we want
// the right-most bit to be captured in carry, in just the same way
// we want the left-most bit to be captured in carry for ASL.
MOS_CHECK_Z(result);
// Carry is set to the value of the bit we're "losing" in the shift
// operation
cpu->P &= ~MOS_CARRY;
if (oper & 0x1) {
cpu->P |= MOS_CARRY;
}
if (cpu->eff_addr) {
mos6502_set(cpu, cpu->eff_addr, result & 0xff);
mos6502_set(cpu, cpu->eff_addr, result);
} else {
cpu->A = result & 0xff;
cpu->A = result;
}
}
@ -141,10 +139,8 @@ DEFINE_INST(lsr)
*/
DEFINE_INST(ora)
{
SET_RESULT(cpu->A | oper);
mos6502_modify_status(cpu, MOS_NZ, cpu->A, result);
cpu->A = result & 0xff;
MOS_CHECK_NZ(cpu->A | oper);
cpu->A |= oper;
}
/*
@ -154,7 +150,7 @@ DEFINE_INST(ora)
*/
DEFINE_INST(rol)
{
SET_RESULT(oper << 1);
vm_8bit result = oper << 1;
// Rotations are effectively _9-bit_. So we aren't rotating bit 7
// into bit 0; we're rotating bit 7 into the carry bit, and we're
@ -168,12 +164,12 @@ DEFINE_INST(rol)
cpu->P |= MOS_CARRY;
}
mos6502_modify_status(cpu, MOS_NZ, oper, result);
MOS_CHECK_NZ(result);
if (cpu->eff_addr) {
mos6502_set(cpu, cpu->eff_addr, result & 0xff);
mos6502_set(cpu, cpu->eff_addr, result);
} else {
cpu->A = result & 0xff;
cpu->A = result;
}
}
@ -183,7 +179,7 @@ DEFINE_INST(rol)
*/
DEFINE_INST(ror)
{
SET_RESULT(oper >> 1);
vm_8bit result = oper >> 1;
// See the code for ROL for my note on 9-bit rotation (vs. 8-bit).
if (cpu->P & MOS_CARRY) {
@ -195,12 +191,12 @@ DEFINE_INST(ror)
cpu->P |= MOS_CARRY;
}
mos6502_modify_status(cpu, MOS_NZ, oper, result);
MOS_CHECK_NZ(result);
if (cpu->eff_addr) {
mos6502_set(cpu, cpu->eff_addr, result & 0xff);
mos6502_set(cpu, cpu->eff_addr, result);
} else {
cpu->A = result & 0xff;
cpu->A = result;
}
}

30
src/mos6502.loadstor.c
View File

@ -14,7 +14,7 @@
*/
DEFINE_INST(lda)
{
mos6502_modify_status(cpu, MOS_NZ, cpu->A, oper);
MOS_CHECK_NZ(oper);
cpu->A = oper;
}
@ -23,7 +23,7 @@ DEFINE_INST(lda)
*/
DEFINE_INST(ldx)
{
mos6502_modify_status(cpu, MOS_NZ, cpu->X, oper);
MOS_CHECK_NZ(oper);
cpu->X = oper;
}
@ -32,7 +32,7 @@ DEFINE_INST(ldx)
*/
DEFINE_INST(ldy)
{
mos6502_modify_status(cpu, MOS_NZ, cpu->Y, oper);
MOS_CHECK_NZ(oper);
cpu->Y = oper;
}
@ -74,9 +74,9 @@ DEFINE_INST(phy)
*/
DEFINE_INST(pla)
{
SET_RESULT(mos6502_pop_stack(cpu));
vm_8bit result = mos6502_pop_stack(cpu);
mos6502_modify_status(cpu, MOS_NZ, cpu->A, result);
MOS_CHECK_NZ(result);
cpu->A = result;
}
@ -93,9 +93,9 @@ DEFINE_INST(plp)
*/
DEFINE_INST(plx)
{
SET_RESULT(mos6502_pop_stack(cpu));
vm_8bit result = mos6502_pop_stack(cpu);
mos6502_modify_status(cpu, MOS_NZ, cpu->X, result);
MOS_CHECK_NZ(result);
cpu->X = result;
}
@ -104,9 +104,9 @@ DEFINE_INST(plx)
*/
DEFINE_INST(ply)
{
SET_RESULT(mos6502_pop_stack(cpu));
vm_8bit result = mos6502_pop_stack(cpu);
mos6502_modify_status(cpu, MOS_NZ, cpu->Y, result);
MOS_CHECK_NZ(result);
cpu->Y = result;
}
@ -149,7 +149,7 @@ DEFINE_INST(stz)
*/
DEFINE_INST(tax)
{
mos6502_modify_status(cpu, MOS_NZ, cpu->X, cpu->A);
MOS_CHECK_NZ(cpu->A);
cpu->X = cpu->A;
}
@ -158,7 +158,7 @@ DEFINE_INST(tax)
*/
DEFINE_INST(tay)
{
mos6502_modify_status(cpu, MOS_NZ, cpu->Y, cpu->A);
MOS_CHECK_NZ(cpu->A);
cpu->Y = cpu->A;
}
@ -167,7 +167,7 @@ DEFINE_INST(tay)
*/
DEFINE_INST(tsx)
{
mos6502_modify_status(cpu, MOS_NZ, cpu->X, cpu->S);
MOS_CHECK_NZ(cpu->S);
cpu->X = cpu->S;
}
@ -176,7 +176,7 @@ DEFINE_INST(tsx)
*/
DEFINE_INST(txa)
{
mos6502_modify_status(cpu, MOS_NZ, cpu->A, cpu->X);
MOS_CHECK_NZ(cpu->X);
cpu->A = cpu->X;
}
@ -185,7 +185,7 @@ DEFINE_INST(txa)
*/
DEFINE_INST(txs)
{
mos6502_modify_status(cpu, MOS_NZ, cpu->S, cpu->X);
MOS_CHECK_NZ(cpu->X);
cpu->S = cpu->X;
}
@ -194,6 +194,6 @@ DEFINE_INST(txs)
*/
DEFINE_INST(tya)
{
mos6502_modify_status(cpu, MOS_NZ, cpu->A, cpu->Y);
MOS_CHECK_NZ(cpu->Y);
cpu->A = cpu->Y;
}