diff options
Diffstat (limited to 'emu/6502.c')
| -rw-r--r-- | emu/6502.c | 256 |
1 files changed, 256 insertions, 0 deletions
diff --git a/emu/6502.c b/emu/6502.c new file mode 100644 index 0000000..0bb1993 --- /dev/null +++ b/emu/6502.c @@ -0,0 +1,256 @@ +#include "6502.h" +#include "memory.h" + +#include <stdio.h> +#include <stdlib.h> + +static const uint16_t reset_vector = 0xFFFC; +static const uint16_t ZP_base = 0x0; +static const uint16_t SP_base = 0x100; + +void cpu_6502_init( cpu_6502_t *cpu, struct memory_t *memory ) +{ + cpu->memory = memory; + cpu->debug = false; +} + +uint16_t cpu_6502_read_word( cpu_6502_t *cpu, uint16_t addr ) +{ + return cpu_6502_read_byte( cpu, addr ) + ( cpu_6502_read_byte( cpu, addr + 1 ) << 8 ); +} + +void cpu_6502_push_byte( cpu_6502_t *cpu, uint8_t data ) +{ + cpu_6502_write_byte( cpu, SP_base + cpu->SP, data ); + cpu->SP--; +} + +void cpu_6502_push_word( cpu_6502_t *cpu, uint16_t data ) +{ + cpu_6502_push_byte( cpu, ( data >> 8 ) & 0xFF ); + cpu_6502_push_byte( cpu, data & 0xFF ); +} + +uint8_t cpu_6502_pop_byte( cpu_6502_t *cpu ) +{ + uint8_t data; + + cpu->SP++; + data = cpu_6502_read_byte( cpu, SP_base + cpu->SP ); + + return data; +} + +uint16_t cpu_6502_pop_word( cpu_6502_t *cpu ) +{ + uint8_t low; + uint8_t high; + uint16_t data; + + low = cpu_6502_pop_byte( cpu ); + high = cpu_6502_pop_byte( cpu ); + + data = ( high << 8 ) + low; + + return data; +} + +void cpu_6502_reset( cpu_6502_t *cpu ) +{ + cpu->PC = cpu_6502_read_word( cpu, reset_vector ); +} + +uint8_t cpu_6502_read_byte( cpu_6502_t *cpu, uint16_t addr ) +{ + return cpu->memory->read( cpu->memory, addr ); +} + +void cpu_6502_write_byte( cpu_6502_t *cpu, uint16_t addr, uint8_t data ) +{ + cpu->memory->write( cpu->memory, addr, data ); +} + +void cpu_6502_write_word( cpu_6502_t *cpu, uint16_t addr, uint16_t data ) +{ + cpu->memory->write( cpu->memory, addr, ( data && 0x00FF ) ); + cpu->memory->write( cpu->memory, addr + 1, ( data && 0x00FF ) ); +} + +void cpu_6502_run( cpu_6502_t *cpu ) +{ + while( true ) { + cpu_6502_step( cpu ); + } +} + +static void handle_zero( cpu_6502_t *cpu, uint8_t x ) +{ + if( x == 0 ) { + cpu->PS |= PS_Z; + } else { + cpu->PS &= ~PS_Z; + } +} + +static void handle_sign( cpu_6502_t *cpu, uint8_t x ) +{ + if( x & 0x80 ) { + cpu->PS |= PS_N; + } else { + cpu->PS &= ~ PS_N; + } +} + +static bool is_negative( cpu_6502_t *cpu ) +{ + return cpu->PS & PS_N; +} + +static bool is_overflow( cpu_6502_t *cpu ) +{ + return cpu->PS & PS_V; +} + +static bool is_brk( cpu_6502_t *cpu ) +{ + return cpu->PS & PS_B; +} + +static bool is_decimal( cpu_6502_t *cpu ) +{ + return cpu->PS & PS_D; +} + +static bool is_interrupt( cpu_6502_t *cpu ) +{ + return cpu->PS & PS_I; +} + +static bool is_zero( cpu_6502_t *cpu ) +{ + return cpu->PS & PS_Z; +} + +static bool is_carry( cpu_6502_t *cpu ) +{ + return cpu->PS & PS_C; +} + +void cpu_6502_print_state( cpu_6502_t *cpu, uint8_t opcode ) +{ + fprintf( stderr, "PC: %04X SP: 01%02X PS: %02X %c%c-%c%c%c%c%c A: %02X X: %02X Y: %02X OP: %02X\n", + cpu->PC, cpu->SP, cpu->PS, + is_negative( cpu ) ? 'N' : 'n', + is_overflow( cpu ) ? 'V' : 'v', + is_brk( cpu ) ? 'B' : 'b', + is_decimal( cpu ) ? 'D' : 'd', + is_interrupt( cpu ) ? 'I' : 'i', + is_zero( cpu ) ? 'Z' : 'z', + is_carry( cpu ) ? 'C' : 'c', + cpu->A, cpu->X, cpu->Y, opcode ); +} + +static void update_negative_and_sign( cpu_6502_t *cpu, uint8_t x ) +{ + handle_zero( cpu, x ); + handle_sign( cpu, x ); +} + +void cpu_6502_print_memory( cpu_6502_t *cpu, uint16_t base, uint8_t size ) +{ + for( uint16_t addr = base; addr <= base + size; addr++ ) { + fprintf( stderr, " %02X", cpu_6502_read_byte( cpu, addr ) ); + } + fputs( "\n", stderr ); +} + +void cpu_6502_print_stack( cpu_6502_t *cpu ) +{ + cpu_6502_print_memory( cpu, SP_base, 255 ); +} + +void cpu_6502_print_zerop_page( cpu_6502_t *cpu ) +{ + cpu_6502_print_memory( cpu, ZP_base, 255 ); +} + +void cpu_6502_step( cpu_6502_t *cpu ) +{ + uint8_t opcode; + uint8_t operand8; + + opcode = cpu_6502_read_byte( cpu, cpu->PC ); + cpu->PC++; + + if( cpu->debug ) { + cpu_6502_print_state( cpu, opcode ); + cpu_6502_print_stack( cpu ); + cpu_6502_print_zerop_page( cpu ); + } + + switch( opcode ) { + case LDX_IMM: + operand8 = cpu_6502_read_byte( cpu, cpu->PC ); + cpu->PC++; + cpu->X = operand8; + update_negative_and_sign( cpu, cpu->X ); + break; + + case LDX_ZERO: + operand8 = cpu_6502_read_byte( cpu, cpu->PC ); + cpu->PC++; + cpu->X = cpu_6502_read_byte( cpu, operand8 ); + update_negative_and_sign( cpu, cpu->X ); + break; + + case LDY_IMM: + operand8 = cpu_6502_read_byte( cpu, cpu->PC ); + cpu->PC++; + cpu->Y = operand8; + update_negative_and_sign( cpu, cpu->Y ); + break; + + case LDY_ZERO: + operand8 = cpu_6502_read_byte( cpu, cpu->PC ); + cpu->PC++; + cpu->Y = cpu_6502_read_byte( cpu, operand8 ); + update_negative_and_sign( cpu, cpu->Y ); + break; + + case LDA_IMM: + operand8 = cpu_6502_read_byte( cpu, cpu->PC ); + cpu->PC++; + cpu->A = operand8; + update_negative_and_sign( cpu, cpu->A ); + break; + + case LDA_ZERO: + operand8 = cpu_6502_read_byte( cpu, cpu->PC ); + cpu->PC++; + cpu->A = cpu_6502_read_byte( cpu, operand8 ); + update_negative_and_sign( cpu, cpu->A ); + break; + + case JMP_ABS: + cpu->PC = cpu_6502_read_word( cpu, cpu->PC ); + break; + + case JSR_ABS: + cpu_6502_push_word( cpu, cpu->PC + 1 ); + cpu->PC = cpu_6502_read_word( cpu, cpu->PC ); + break; + + case RTS_IMPL: + cpu->PC = cpu_6502_pop_word( cpu ); + cpu->PC++; + break; + + case TXS_IMPL: + cpu->SP = cpu->X; + break; + + default: + fprintf( stderr, "ERROR: Illegal opcode %02X at PC %04X\n", opcode, cpu->PC ); + exit( EXIT_FAILURE ); + } +} |