criscv/src/instruction_executor.c

#include "cpu.h"
#include <stdio.h>

static void printBinary(int num) {
    if (num > 1) {
        printBinary(num / 2);
    }
    printf("%d", num % 2);
}

int execute_instruction_on_cpu(CPU *cpu, uint32_t instruction) {
    AddressSpace *addressSpace = cpu->addressSpace;
    uint32_t *registers = cpu->registers;
    registers[0] = 0; // x0 is always 0

    uint8_t opcode = instruction & 0x7F;
    uint8_t rd = instruction >> 7 & 0x1F;

    printf("\nPC: %d\n", cpu->programCounter);

    if (opcode != 0) {
        printf("Instruction: ");
        printBinary(instruction);
        printf("\nOpcode: 0x%X (", opcode);
        printBinary(opcode);
        printf(")\n");
        printf("Dest. reg.: x%d\n", rd);
    }

    switch (opcode) {
        case 0b0010011: { // OP-IMM for Integer Register-Immediate Instructions (I type)
            uint8_t funct3 = instruction >> 12 & 0x7;
            uint8_t rs1 = instruction >> 15 & 0x1F;
            uint32_t imm = (int32_t)instruction >> 20;

            switch (funct3) {
                case 0b000: // ADDI
                    registers[rd] = registers[rs1] + imm;
                    break;
                default:
                    // TODO other and illegal instruction
                    break;
            }
            /* code */
            break;
        }
        case 0b0110111: { // LUI load upper immediate (U type)
            uint32_t imm = instruction & 0xFFFFF000;
            registers[rd] = imm;
            break;
        }
        case 0b1100111: { // JALR for unconditional jump and link register (I type)
            uint8_t funct3 = instruction >> 12 & 0x7;

            if (funct3 != 0) {
                // TODO illegal instruction
            }

            uint8_t rs1 = instruction >> 15 & 0x1F;
            uint32_t imm = (int32_t)instruction >> 20;

            int result = registers[rs1] + imm;
            result &= ~1; // set LSB to 0

            registers[rd] = cpu->programCounter + 4;
            cpu->programCounter = result - 4; // program counter is always incremented after executing instruction

            break;
        }
        default:
            // TODO illegal instruction
            break;
    }

    // TODO

    return 0;
}