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Fix stack in several ways

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1. The stack should descend, not ascend;
2. The stack should be pushed byte for byte; meaning, when pushing P or
A, those should consume one byte, not two;
3. The MSB should be pushed first when doing JSR and BRK, which makes
some sense if you were reading the stack from $0100 - $01FF.
This commit is contained in:
Peter Evans 2018-01-21 01:09:42 -06:00
parent 42c7fcbb47
commit 0581ed6b44
4 changed files with 18 additions and 12 deletions
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4
include/mos6502.h
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@ -136,13 +136,13 @@ extern int mos6502_instruction(vm_8bit);
extern mos6502 *mos6502_create(vm_segment *, vm_segment *);
extern mos6502_instruction_handler mos6502_get_instruction_handler(vm_8bit);
extern vm_16bit mos6502_get16(mos6502 *, size_t);
extern vm_16bit mos6502_pop_stack(mos6502 *);
extern vm_8bit mos6502_get(mos6502 *, size_t);
extern vm_8bit mos6502_pop_stack(mos6502 *);
extern void mos6502_execute(mos6502 *);
extern void mos6502_free(mos6502 *);
extern void mos6502_last_executed(mos6502 *, vm_8bit *, vm_8bit *, vm_16bit *);
extern void mos6502_modify_status(mos6502 *, vm_8bit, int, int);
extern void mos6502_push_stack(mos6502 *, vm_16bit);
extern void mos6502_push_stack(mos6502 *, vm_8bit);
extern void mos6502_set(mos6502 *, size_t, vm_8bit);
extern void mos6502_set16(mos6502 *, size_t, vm_16bit);
extern void mos6502_set_memory(mos6502 *, vm_segment *, vm_segment *);

2
src/apple2.c
View File

@ -254,7 +254,7 @@ apple2_reset(apple2 *mach)
{
mach->cpu->P = MOS_INTERRUPT;
mach->cpu->PC = vm_segment_get16(mach->main, 0xFFFC);
mach->cpu->S = 0;
mach->cpu->S = 0xff;
// Switch video mode back to 40 column text
apple2_set_display(mach, DISPLAY_DEFAULT);

14
src/mos6502.c
View File

@ -163,7 +163,7 @@ mos6502_create(vm_segment *rmem, vm_segment *wmem)
cpu->X = 0;
cpu->Y = 0;
cpu->P = 0;
cpu->S = 0;
cpu->S = 0xff;
return cpu;
}
@ -189,29 +189,29 @@ mos6502_free(mos6502 *cpu)
* 256, addresses.
*/
void
mos6502_push_stack(mos6502 *cpu, vm_16bit addr)
mos6502_push_stack(mos6502 *cpu, vm_8bit addr)
{
mos6502_set16(cpu, 0x100 + cpu->S, addr);
mos6502_set(cpu, 0x100 + cpu->S, addr);
// And finally we need to increment S by 2 (since we've used two
// bytes in the stack).
cpu->S += 2;
cpu->S--;
}
/*
* Pop an address from the stack and return that.
*/
vm_16bit
vm_8bit
mos6502_pop_stack(mos6502 *cpu)
{
// The first thing we want to do here is to decrement S by 2, since
// the value we want to return is two positions back.
cpu->S -= 2;
cpu->S++;
// We need to use a bitwise-or operation to combine the hi and lo
// bytes we retrieve from the stack into the actual position we
// would use for the PC register.
return mos6502_get16(cpu, 0x0100 + cpu->S);
return mos6502_get(cpu, 0x0100 + cpu->S);
}
/*

10
src/mos6502.exec.c
View File

@ -14,7 +14,8 @@
*/
DEFINE_INST(brk)
{
mos6502_push_stack(cpu, cpu->PC);
mos6502_push_stack(cpu, cpu->PC >> 8);
mos6502_push_stack(cpu, cpu->PC & 0xff);
mos6502_push_stack(cpu, cpu->P);
cpu->P |= MOS_INTERRUPT;
cpu->PC += 2;
@ -36,7 +37,10 @@ DEFINE_INST(jmp)
*/
DEFINE_INST(jsr)
{
mos6502_push_stack(cpu, cpu->PC + 3);
vm_16bit pc3 = cpu->PC + 3;
mos6502_push_stack(cpu, pc3 >> 8);
mos6502_push_stack(cpu, pc3 & 0xff);
cpu->PC = cpu->eff_addr;
}
@ -57,6 +61,7 @@ DEFINE_INST(rti)
{
cpu->P = mos6502_pop_stack(cpu);
cpu->PC = mos6502_pop_stack(cpu);
cpu->PC |= mos6502_pop_stack(cpu) << 8;
}
/*
@ -66,4 +71,5 @@ DEFINE_INST(rti)
DEFINE_INST(rts)
{
cpu->PC = mos6502_pop_stack(cpu);
cpu->PC |= mos6502_pop_stack(cpu) << 8;
}