lc3.c

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <signal.h>
/* unix */
#include <unistd.h>

#include <sys/time.h>
#include <sys/termios.h>
#include <sys/mman.h>

/* Registers */
enum {
    R_R0 = 0,
    R_R1,
    R_R2,
    R_R3,
    R_R4,
    R_R5,
    R_R6,
    R_R7,
    R_PC,  /* program counter */
    R_COND,
    R_COUNT
};

/* Opcodes */
enum {
    OP_BR = 0, /* branch */
    OP_ADD,    /* add  */
    OP_LD,     /* load */
    OP_ST,     /* store */
    OP_JSR,    /* jump to register */
    OP_AND,    /* bitwise and */
    OP_LDR,    /* load register */
    OP_STR,    /* store register */
    OP_RTI,    /* unused */
    OP_NOT,    /* bitwise not */
    OP_LDI,    /* load indirect */
    OP_STI,    /* store indirect */
    OP_JMP,    /* jump */
    OP_RES,    /* reserved (unused) */
    OP_LEA,    /* load effective address */
    OP_TRAP    /* execute trap */
};

/* Condition Flags */
enum {
    FL_POS = 1 << 0,  /* P */
    FL_ZRO = 1 << 1,  /* Z */
    FL_NEG = 1 << 2,  /* N */
};

/* Memory Mapped Registers */
enum {
    MR_KBSR = 0xFE00, /* keyboard status */
    MR_KBDR = 0xFE02  /* keyboard data */
};

/* TRAP Codes */
enum {
    TRAP_GETC = 0x20,   /* get character from keyboard, not echoed onto the terminal */
    TRAP_OUT = 0x21,    /* output a character */
    TRAP_PUTS = 0x22,   /* output a word string */
    TRAP_IN = 0x23,     /* get character from keyboard, echoed onto the terminal */
    TRAP_PUTSP = 0x24,  /* output a byte string */
    TRAP_HALT = 0x25    /* halt the program */
};

/* Memory Storage */
/* 65536 locations */
uint16_t memory[UINT16_MAX];

/* Register Storage */
uint16_t reg[R_COUNT];

/* Sign Extend */
uint16_t sign_extend(uint16_t x, int bit_count) {
    if ((x >> (bit_count - 1)) & 1) {
        x |= (0xFFFF << bit_count);
    }
    return x;
}

/* Swap */
uint16_t swap16(uint16_t x) {
    return (x << 8) | (x >> 8);
}

/* Update Flags */
void update_flags(uint16_t r) {
    if (reg[r] == 0) {
        reg[R_COND] = FL_ZRO;
    } else if (reg[r] >> 15) {
        /* a 1 in the left-most bit indicates negative */
        reg[R_COND] = FL_NEG;
    } else {
        reg[R_COND] = FL_POS;
    }
}

/* Read Image File */
void read_image_file(FILE *file) {
    /* the origin tells us where in memory to place the image */
    uint16_t origin;
    fread(&origin, sizeof(origin), 1, file);
    origin = swap16(origin);

    /* we know the maximum file size so we only need one fread */
    uint16_t max_read = UINT16_MAX - origin;
    uint16_t *p = memory + origin;
    size_t read = fread(p, sizeof(uint16_t), max_read, file);

    /* swap to little endian */
    while (read-- > 0) {
        *p = swap16(*p);
        ++p;
    }
}

/* Read Image */
int read_image(const char *image_path) {
    FILE *file = fopen(image_path, "rb");
    if (!file) { return 0; };
    read_image_file(file);
    fclose(file);
    return 1;
}

/* Check Key */
uint16_t check_key() {
    fd_set read_fds;
    FD_ZERO(&read_fds);
    FD_SET(STDIN_FILENO, &read_fds);

    struct timeval timeout;
    timeout.tv_sec = 0;
    timeout.tv_usec = 0;
    return select(1, &read_fds, NULL, NULL, &timeout) != 0;
}

void mem_write(uint16_t address, uint16_t val) {
    memory[address] = val;
}

uint16_t mem_read(uint16_t address) {
    if (address == MR_KBSR) {
        if (check_key()) {
            memory[MR_KBSR] = (1 << 15);
            memory[MR_KBDR] = getchar();
        } else {
            memory[MR_KBSR] = 0;
        }
    }
    return memory[address];
}

/* Input Buffering */
struct termios original_tio;

void disable_input_buffering() {
    tcgetattr(STDIN_FILENO, &original_tio);
    struct termios new_tio = original_tio;
    new_tio.c_lflag &= ~ICANON & ~ECHO;
    tcsetattr(STDIN_FILENO, TCSANOW, &new_tio);
}

void restore_input_buffering() {
    tcsetattr(STDIN_FILENO, TCSANOW, &original_tio);
}

/* Handle Interrupt */
void handle_interrupt(int signal) {
    restore_input_buffering();
    printf("\n");
    exit(-2);
}

int main(int argc, const char *argv[]) {
    /* Load Arguments */
    if (argc < 2) {
        /* show usage string */
        printf("lc3 <image-file1> ...\n");
        exit(2);
    }

    for (int i = 0; i < argc; ++i) {
        if (!read_image(argv[i])) {
            printf("failed to load image: %s\n", argv[i]);
            exit(1);
        }
    }

    /* Setup */
    signal(SIGINT, handle_interrupt);
    disable_input_buffering();

    /* set the PC to starting position */
    /* 0x3000 is the default */
    enum {
        PC_START = 0x3000
    };
    reg[R_PC] = PC_START;

    int running = 1;
    while (running) {
        /* FETCH */
        uint16_t instr = mem_read(reg[R_PC]++);
        uint16_t op = instr >> 12;

        switch (op) {
            case OP_ADD: {
                /* destination register (DR) */
                uint16_t r0 = (instr >> 9) & 0x7;
                /* first operand (SR1) */
                uint16_t r1 = (instr >> 6) & 0x7;
                /* whether we are in immediate mode */
                uint16_t imm_flag = (instr >> 5) & 0x1;

                if (imm_flag) {
                    uint16_t imm5 = sign_extend(instr & 0x1F, 5);
                    reg[r0] = reg[r1] + imm5;
                } else {
                    uint16_t r2 = instr & 0x7;
                    reg[r0] = reg[r1] + reg[r2];
                }

                update_flags(r0);
                break;
            }
            case OP_AND: {
                uint16_t r0 = (instr >> 9) & 0x7;
                uint16_t r1 = (instr >> 6) & 0x7;
                uint16_t imm_flag = (instr >> 5) & 0x1;

                if (imm_flag) {
                    uint16_t imm5 = sign_extend(instr & 0x1F, 5);
                    reg[r0] = reg[r1] & imm5;
                } else {
                    uint16_t r2 = instr & 0x7;
                    reg[r0] = reg[r1] & reg[r2];
                }
                update_flags(r0);
            }
                break;
            case OP_NOT: {
                uint16_t r0 = (instr >> 9) & 0x7;
                uint16_t r1 = (instr >> 6) & 0x7;

                reg[r0] = ~reg[r1];
                update_flags(r0);
            }
                break;
            case OP_BR: {
                uint16_t pc_offset = sign_extend(instr & 0x1FF, 9);
                uint16_t cond_flag = (instr >> 9) & 0x7;
                if (cond_flag & reg[R_COND]) {
                    reg[R_PC] += pc_offset;
                }
            }
                break;
            case OP_JMP: {
                /* Also handles RET */
                uint16_t r1 = (instr >> 6) & 0x7;
                reg[R_PC] = reg[r1];
            }
                break;
            case OP_JSR: {
                uint16_t long_flag = (instr >> 11) & 1;
                reg[R_R7] = reg[R_PC];
                if (long_flag) {
                    uint16_t long_pc_offset = sign_extend(instr & 0x7FF, 11);
                    reg[R_PC] += long_pc_offset;  /* JSR */
                } else {
                    uint16_t r1 = (instr >> 6) & 0x7;
                    reg[R_PC] = reg[r1];  /* JSRR */
                }
                break;
            }
                break;
            case OP_LD: {
                uint16_t r0 = (instr >> 9) & 0x7;
                uint16_t pc_offset = sign_extend(instr & 0x1FF, 9);
                reg[r0] = mem_read(reg[R_PC] + pc_offset);
                update_flags(r0);
            }
                break;
            case OP_LDI: {
                /* destination register (DR) */
                uint16_t r0 = (instr >> 9) & 0x7;
                /* PCoffset 9*/
                uint16_t pc_offset = sign_extend(instr & 0x1FF, 9);
                /* add pc_offset to the current PC, look at that memory location to get the final address */
                reg[r0] = mem_read(mem_read(reg[R_PC] + pc_offset));
                update_flags(r0);
            }
                break;
            case OP_LDR: {
                uint16_t r0 = (instr >> 9) & 0x7;
                uint16_t r1 = (instr >> 6) & 0x7;
                uint16_t offset = sign_extend(instr & 0x3F, 6);
                reg[r0] = mem_read(reg[r1] + offset);
                update_flags(r0);
            }
                break;
            case OP_LEA: {
                uint16_t r0 = (instr >> 9) & 0x7;
                uint16_t pc_offset = sign_extend(instr & 0x1FF, 9);
                reg[r0] = reg[R_PC] + pc_offset;
                update_flags(r0);
            }
                break;
            case OP_ST: {
                uint16_t r0 = (instr >> 9) & 0x7;
                uint16_t pc_offset = sign_extend(instr & 0x1FF, 9);
                mem_write(reg[R_PC] + pc_offset, reg[r0]);
            }
                break;
            case OP_STI: {
                uint16_t r0 = (instr >> 9) & 0x7;
                uint16_t pc_offset = sign_extend(instr & 0x1FF, 9);
                mem_write(mem_read(reg[R_PC] + pc_offset), reg[r0]);
            }
                break;
            case OP_STR: {
                uint16_t r0 = (instr >> 9) & 0x7;
                uint16_t r1 = (instr >> 6) & 0x7;
                uint16_t offset = sign_extend(instr & 0x3F, 6);
                mem_write(reg[r1] + offset, reg[r0]);
            }
                break;
            case OP_TRAP:
                switch (instr & 0xFF) {
                    case TRAP_GETC: {
                        /* read a single ASCII char */
                        reg[R_R0] = (uint16_t) getchar();
                    }

                        break;
                    case TRAP_OUT: {
                        putc((char) reg[R_R0], stdout);
                        fflush(stdout);
                    }

                        break;
                    case TRAP_PUTS: {
                        /* one char per word */
                        uint16_t *c = memory + reg[R_R0];
                        while (*c) {
                            putc((char) *c, stdout);
                            ++c;
                        }
                        fflush(stdout);
                    }

                        break;
                    case TRAP_IN: {
                        printf("Enter a character: ");
                        char c = (char) getchar();
                        putc(c, stdout);
                        reg[R_R0] = (uint16_t) c;
                    }

                        break;
                    case TRAP_PUTSP: {
                        /* one char per byte (two bytes per word)
                           here we need to swap back to
                           big endian format */
                        uint16_t *c = memory + reg[R_R0];
                        while (*c) {
                            char char1 = (char) ((*c) & 0xFF);
                            putc(char1, stdout);
                            char char2 = (char) ((*c) >> 8);
                            if (char2) putc(char2, stdout);
                            ++c;
                        }
                        fflush(stdout);
                    }

                        break;
                    case TRAP_HALT: {
                        puts("HALT");
                        fflush(stdout);
                        running = 0;
                    }

                        break;
                }
                break;
            case OP_RES:
            case OP_RTI:
            default: {
                /* BAD OPCODE */
                abort();
            }
                break;
        }
    }

    /* Shutdown */
    restore_input_buffering();
}