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#include <stdlib.h>
#include <stdint.h>
#include <stdio.h>
#include <assert.h>
struct CPU{
uint8_t register_a;
uint8_t register_x;
uint8_t status;
uint16_t pc;
uint8_t memory[0xFFFF];
};
struct CPU create_cpu();
int array_size(uint8_t *program);
uint8_t mem_read(struct CPU *cpu, uint16_t addr);
void mem_write(struct CPU *cpu, uint16_t addr, uint8_t data);
void load(struct CPU *cpu, uint8_t *program);
void load_and_run(struct CPU *cpu, uint8_t *program);
void reset_cpu(struct CPU *cpu);
void update_zero_and_negative_flags(struct CPU *cpu, uint8_t result);
void lda(struct CPU *cpu, uint8_t value);
void interpret(struct CPU *cpu);
uint16_t mem_read_u16(struct CPU *cpu, uint16_t pos);
struct CPU create_cpu(){
struct CPU cpu = {
.register_a = 0,
.register_x = 0,
.status = 0,
.pc = 0
};
return cpu;
}
int array_size(uint8_t *program){
return sizeof(program) / sizeof(program[0]);
}
uint8_t mem_read(struct CPU *cpu, uint16_t addr){
return cpu->memory[addr];
}
void mem_write(struct CPU *cpu, uint16_t addr, uint8_t data){
cpu->memory[addr] = data;
}
uint16_t mem_read_u16(struct CPU *cpu, uint16_t pos){
//little-endian
uint16_t lo = (uint16_t)mem_read(cpu, pos);
uint16_t hi = (uint16_t)mem_read(cpu, pos+1);
return (hi<<8) | (lo);
}
void mem_write_u16(struct CPU *cpu, uint16_t pos, uint16_t data){
uint8_t hi = (uint8_t)(data>>8);
uint8_t lo = (uint8_t)(data & 0x00ff);
mem_write(cpu, pos, lo);
mem_write(cpu, pos+1, hi);
}
void load(struct CPU *cpu, uint8_t *program){
for(int i=0; i < array_size(program); ++i){
cpu->memory[0x8000+i] = program[i];
}
mem_write_u16(cpu, 0xfffc, 0x8000);
}
void load_and_run(struct CPU *cpu, uint8_t *program){
load(cpu, program);
reset_cpu(cpu);
interpret(cpu);
}
void reset_cpu(struct CPU *cpu){
(*cpu).register_a = 0;
(*cpu).register_x = 0;
(*cpu).status = 0;
(*cpu).pc = mem_read_u16(cpu, 0xfffc);
}
void update_zero_and_negative_flags(struct CPU *cpu, uint8_t result){
if(result == 0)
cpu->status |= 0b00000010;
else
cpu->status &= 0b11111101;
if((result & 0b10000000) != 0)
cpu->status |= 0b10000000;
else
cpu->status &= 0b01111111;
}
void lda(struct CPU *cpu, uint8_t value){
cpu->register_a = value;
update_zero_and_negative_flags(cpu, cpu->register_a);
}
void interpret(struct CPU *cpu){
while(1){
uint8_t opcode = mem_read(cpu, cpu->pc);
cpu->pc += 1;
switch(opcode){
case 0x00:
return;
case 0xAA:
cpu->register_x = cpu->register_a;
update_zero_and_negative_flags(cpu, cpu->register_x);
break;
case 0xA9:
uint8_t param = mem_read(cpu, cpu->pc);
cpu->pc += 1;
lda(cpu, param);
break;
case 0xE8:
cpu->register_x++;
update_zero_and_negative_flags(cpu, cpu->register_x);
break;
default:
break;
}
}
return;
}
int main(){
printf("test_0xa9_lda_immediate_load_data: ");
struct CPU cpu_1 = create_cpu();
uint8_t program_1[] = {0xa9, 0x05, 0x00};
load_and_run(&cpu_1, program_1);
assert(cpu_1.register_a == 0x05);
assert((cpu_1.status & 0b00000010) == 0b00);
assert((cpu_1.status & 0b10000000) == 0);
printf("PASSED\n");
printf("test_0xa9_lda_zero_flag: ");
struct CPU cpu_2 = create_cpu();
uint8_t program_2[] = {0xa9, 0x00, 0x00};
load_and_run(&cpu_2, program_2);
assert((cpu_2.status & 0b00000010) == 0b00000010);
printf("PASSED\n");
printf("text_0xaa_tax_move_a_to_x: ");
struct CPU cpu_3 = create_cpu();
uint8_t program_3[] = {0xa9, 10, 0xaa, 0x00};
load_and_run(&cpu_3, program_3);
assert(cpu_3.register_x == 10);
printf("PASSED\n");
printf("test_5_ops_working_together: ");
struct CPU cpu_4 = create_cpu();
uint8_t program_4[] = {0xa9, 0xc0, 0xaa, 0xe8, 0x00};
load_and_run(&cpu_4, program_4);
assert(cpu_4.register_x == 0xC1);
printf("PASSED\n");
printf("test_inx_overflow: ");
struct CPU cpu_5 = create_cpu();
uint8_t program_5[] = {0xa9, 0xff, 0xaa, 0xe8, 0xe8, 0x00};
load_and_run(&cpu_5, program_5);
assert(cpu_5.register_x == 1);
printf("PASSED\n");
}
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