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Remove next_byte, read_byte; execute works without an opcode arg

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This also adds RTS and RTI as instructions that "would jump".
This commit is contained in:
Peter Evans 2018-01-09 20:58:19 -06:00
parent a785eb5665
commit 80a7671a19
1 changed files with 17 additions and 36 deletions
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53
src/mos6502.c
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@ -178,21 +178,6 @@ mos6502_free(mos6502 *cpu)
free(cpu); free(cpu);
} }
/*
* Return the next byte from the PC register position, and increment the
* PC register.
*/
vm_8bit
mos6502_next_byte(mos6502 *cpu)
{
vm_8bit byte;
byte = vm_segment_get(cpu->memory, cpu->PC);
cpu->PC++;
return byte;
}
/* /*
* Push a _16-bit_ number to the stack. Generally speaking, only * Push a _16-bit_ number to the stack. Generally speaking, only
* addresses are pushed to the stack, such that would be contained in * addresses are pushed to the stack, such that would be contained in
@ -349,17 +334,20 @@ mos6502_get_instruction_handler(vm_8bit opcode)
* from soup to nuts. * from soup to nuts.
*/ */
void void
mos6502_execute(mos6502 *cpu, vm_8bit opcode) mos6502_execute(mos6502 *cpu)
{ {
vm_8bit operand = 0; vm_8bit opcode, operand = 0;
int cycles; int cycles, bytes;
mos6502_address_resolver resolver; mos6502_address_resolver resolver;
mos6502_instruction_handler handler; mos6502_instruction_handler handler;
opcode = vm_segment_get(cpu->memory, cpu->PC);
// The disassembler knows how many bytes each operand requires // The disassembler knows how many bytes each operand requires
// (maybe this code doesn't belong in the disassembler); let's use // (maybe this code doesn't belong in the disassembler); let's use
// that to figure out the total number of bytes to skip. // that to figure out the total number of bytes to skip. We add 1
mos6502_dis_expected_bytes(mos6502_addr_mode(opcode)); // because we need to account for the opcode as well.
bytes = 1 + mos6502_dis_expected_bytes(mos6502_addr_mode(opcode));
// First, we need to know how to resolve our effective address and // First, we need to know how to resolve our effective address and
// how to execute anything. // how to execute anything.
@ -393,6 +381,12 @@ mos6502_execute(mos6502 *cpu, vm_8bit opcode)
// programs to feel faster or slower in relation to that. // programs to feel faster or slower in relation to that.
cycles = mos6502_cycles(cpu, opcode); cycles = mos6502_cycles(cpu, opcode);
// If we need to jump, then the handler has to take care of updating
// PC. If not, then we need to do it.
if (!mos6502_would_jump(mos6502_instruction(opcode))) {
cpu->PC += bytes;
}
// FIXME: uh this probably isn't right, but I wanted to do // FIXME: uh this probably isn't right, but I wanted to do
// something. // something.
usleep(cycles * 100000); usleep(cycles * 100000);
@ -401,21 +395,6 @@ mos6502_execute(mos6502 *cpu, vm_8bit opcode)
return; return;
} }
/*
* Return the next byte in memory according to the program counter
* register, and then increment the register.
*/
vm_8bit
mos6502_read_byte(mos6502 *cpu)
{
vm_8bit byte;
byte = vm_segment_get(cpu->memory, cpu->PC);
cpu->PC++;
return byte;
}
/* /*
* Return true if the given instruction would require that we jump * Return true if the given instruction would require that we jump
* to somewhere else in the program. * to somewhere else in the program.
@ -434,7 +413,9 @@ mos6502_would_jump(int inst_code)
inst_code == BVC || inst_code == BVC ||
inst_code == BVS || inst_code == BVS ||
inst_code == JMP || inst_code == JMP ||
inst_code == JSR; inst_code == JSR ||
inst_code == RTS ||
inst_code == RTI;
} }
/* /*