zba/src/cpu.zig

const std = @import("std");
const Bus = @import("bus.zig").Bus;
const Scheduler = @import("scheduler.zig").Scheduler;

const comptimeDataProcessing = @import("cpu/data_processing.zig").comptimeDataProcessing;
const comptimeSingleDataTransfer = @import("cpu/single_data_transfer.zig").comptimeSingleDataTransfer;
const comptimeHalfSignedDataTransfer = @import("cpu/half_signed_data_transfer.zig").comptimeHalfSignedDataTransfer;

pub const InstrFn = fn (*ARM7TDMI, *Bus, u32) void;
const ARM_LUT: [0x1000]InstrFn = populate();

pub const ARM7TDMI = struct {
    r: [16]u32,
    sch: *Scheduler,
    bus: *Bus,
    cpsr: CPSR,

    pub fn new(scheduler: *Scheduler, bus: *Bus) @This() {
        const cpsr: u32 = 0x0000_00DF;
        return .{
            .r = [_]u32{0x00} ** 16,
            .sch = scheduler,
            .bus = bus,
            .cpsr = @bitCast(CPSR, cpsr),
        };
    }

    pub inline fn step(self: *@This()) u64 {
        const opcode = self.fetch();

        std.debug.print("R15: 0x{X:}\n", .{opcode}); // Debug

        ARM_LUT[armIdx(opcode)](self, self.bus, opcode);

        return 1;
    }

    fn fetch(self: *@This()) u32 {
        const word = self.bus.readWord(self.r[15]);
        self.r[15] += 4;
        return word;
    }

    fn fakePC(self: *const @This()) u32 {
        return self.r[15] + 4;
    }
};

fn armIdx(opcode: u32) u12 {
    return @truncate(u12, opcode >> 20 & 0xFF) << 4 | @truncate(u12, opcode >> 8 & 0xF);
}

fn populate() [0x1000]InstrFn {
    return comptime {
        @setEvalBranchQuota(0x5000);
        var lut = [_]InstrFn{undefined_instr} ** 0x1000;

        var i: usize = 0;
        while (i < lut.len) : (i += 1) {
            if (i >> 10 & 0x3 == 0b00) {
                const I = i >> 9 & 0x01 == 0x01;
                const S = i >> 4 & 0x01 == 0x01;
                const instrKind = i >> 5 & 0x0F;

                lut[i] = comptimeDataProcessing(I, S, instrKind);
            }

            if (i >> 9 & 0x7 == 0b000 and i >> 6 & 0x01 == 0x00 and i & 0xF == 0x0) {
                // Halfword and Signed Data Transfer with register offset
                const P = i >> 8 & 0x01 == 0x01;
                const U = i >> 7 & 0x01 == 0x01;
                const I = true;
                const W = i >> 5 & 0x01 == 0x01;
                const L = i >> 4 & 0x01 == 0x01;

                lut[i] = comptimeHalfSignedDataTransfer(P, U, I, W, L);
            }

            if (i >> 9 & 0x7 == 0b000 and i >> 6 & 0x01 == 0x01) {
                // Halfword and Signed Data Tranfer with immediate offset
                const P = i >> 8 & 0x01 == 0x01;
                const U = i >> 7 & 0x01 == 0x01;
                const I = false;
                const W = i >> 5 & 0x01 == 0x01;
                const L = i >> 4 & 0x01 == 0x01;

                lut[i] = comptimeHalfSignedDataTransfer(P, U, I, W, L);
            }

            if (i >> 10 & 0x3 == 0b01 and i & 0x01 == 0x00) {
                const I = i >> 9 & 0x01 == 0x01;
                const P = i >> 8 & 0x01 == 0x01;
                const U = i >> 7 & 0x01 == 0x01;
                const B = i >> 6 & 0x01 == 0x01;
                const W = i >> 5 & 0x01 == 0x01;
                const L = i >> 4 & 0x01 == 0x01;

                lut[i] = comptimeSingleDataTransfer(I, P, U, B, W, L);
            }

            if (i >> 9 & 0x7 == 0b101) {
                const L = i >> 8 & 0x01 == 0x01;
                lut[i] = comptimeBranch(L);
            }
        }

        return lut;
    };
}

const CPSR = packed struct {
    n: bool, // Negative / Less Than
    z: bool, // Zero
    c: bool, // Carry / Borrow / Extend
    v: bool, // Overflow
    _: u20,
    i: bool, // IRQ Disable
    f: bool, // FIQ Diable
    t: bool, // State
    m: Mode, // Mode
};

const Mode = enum(u5) {
    User = 0b10000,
    Fiq = 0b10001,
    Irq = 0b10010,
    Supervisor = 0b10011,
    Abort = 0b10111,
    Undefined = 0b11011,
    System = 0b11111,
};

fn undefined_instr(_: *ARM7TDMI, _: *Bus, opcode: u32) void {
    const id = armIdx(opcode);
    std.debug.panic("[0x{X:}] 0x{X:} is an illegal opcode", .{ id, opcode });
}

fn comptimeBranch(comptime L: bool) InstrFn {
    return struct {
        fn branch(cpu: *ARM7TDMI, _: *Bus, opcode: u32) void {
            if (L) {
                cpu.r[14] = cpu.r[15] - 4;
            }

            const offset = @bitCast(i32, (opcode << 2) << 8) >> 8;
            cpu.r[15] = cpu.fakePC() + @bitCast(u32, offset);
        }
    }.branch;
}