generate.c

/* Microprogram generator
 * -----------------------
 * This program is used to generate the
 * microprogram data that will be stored
 * on the microprogram ROM of the CPU.
 * You can view the microprogrammed machine
 * as an abstract moore machine. Every state
 * has a specific value for the output and
 * a set of the ordered pairs: (input_value,
 * next_state_val);
 */

#include <stdio.h>

#define     A       0
#define     B       1
#define     C       2

typedef struct state {
    int next_state[2];
    int output;
} state_t;

int main() {

    // A machine that outputs 1 when
    // number of received 1s is divisible by 3.
    // The abstract machine consists of 3 states:
    // P.S |    N.S    |  OUTPUT  |    NOTE
    //     | X=0   X=1 |          |
    // ----|-----------|----------|--------------
    //  A  |  A     B  |    1     | n mode 3 = 0
    //  B  |  B     C  |    0     | n mode 3 = 1
    //  C  |  C     A  |    0     | n mode 3 = 2

    // define the Moore machine:
    state_t machine[3];

    // define the loop counters:
    int i, j;

    // prepare the ROM image:
    FILE *rom = fopen("image.bin", "w");

    // define state A:
    machine[A].next_state[0] = A;
    machine[A].next_state[1] = B;
    machine[A].output = 1;

    // define state B:
    machine[B].next_state[0] = B;
    machine[B].next_state[1] = C;
    machine[B].output = 0;

    // define state C:
    machine[C].next_state[0] = C;
    machine[C].next_state[1] = A;
    machine[C].output = 0;

    // The layout of the ROM will be like this:
    //          ADD |  N/A  | O/P | NS
    //          --- | ----- | --- | --
    // State A: 000 | 00000 |  1  | 00
    //          001 | 00000 |  1  | 01
    // State B: 010 | 00000 |  0  | 01
    //          011 | 00000 |  0  | 10
    // State C: 100 | 00000 |  0  | 10
    //          101 | 00000 |  0  | 00
    // NS is connected to the higher two bits of the
    // 3-bit address bus through a clocked latch.
    // The lower bit of the address is directly
    // connected to the input.

    // now generate the microprogram:
    for (i = 0; i < 3; i++) {
        for (j = 0; j < 2; j++) {
            char rombyte = machine[i].next_state[j] |
                           (machine[i].output << 2);
            fputc(rombyte, rom);
        }
    }

    // pad 2 extra characters to make it 8-bit:
    fputc(0, rom);
    fputc(0, rom);

    // close the file:
    fclose(rom);

    // done:
    return 0;

}