84 lines
2.5 KiB
Zig
84 lines
2.5 KiB
Zig
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const std = @import("std");
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const arm = @import("../cpu.zig");
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const Arm7tdmi = arm.Arm7tdmi;
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const CPSR = arm.CPSR;
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pub inline fn exec(cpu: *Arm7tdmi, opcode: u32) u32 {
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var shift_amt: u8 = undefined;
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if (opcode >> 4 & 1 == 1) {
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shift_amt = @truncate(u8, cpu.r[opcode >> 8 & 0xF]);
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} else {
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shift_amt = @truncate(u8, opcode >> 7 & 0x1F);
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}
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const rm = cpu.r[opcode & 0xF];
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return switch (@truncate(u2, opcode >> 5)) {
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0b00 => logical_left(&cpu.cpsr, rm, shift_amt),
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0b01 => logical_right(&cpu.cpsr, rm, shift_amt),
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0b10 => arithmetic_right(&cpu.cpsr, rm, shift_amt),
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0b11 => rotate_right(&cpu.cpsr, rm, shift_amt),
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};
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}
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pub inline fn logical_left(cpsr: *CPSR, rm: u32, shift_byte: u8) u32 {
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const shift_amt = @truncate(u5, shift_byte);
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const bit_count: u8 = @typeInfo(u32).Int.bits;
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var result: u32 = 0x0000_0000;
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if (shift_byte < bit_count) {
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// We can perform a well-defined shift here
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// FIXME: We assume cpu.r[rs] == 0 and imm_shift == 0 are equivalent
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if (shift_amt != 0) {
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const carry_bit = @truncate(u5, bit_count - shift_amt);
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cpsr.c.write(rm >> carry_bit & 1 == 1);
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}
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result = rm << shift_amt;
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} else if (shift_byte == bit_count) {
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// Shifted all bits out, carry bit is bit 0 of rm
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cpsr.c.write(rm & 1 == 1);
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} else {
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// Shifted all bits out, carry bit has also been shifted out
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cpsr.c.write(false);
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}
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return result;
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}
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pub inline fn logical_right(cpsr: *CPSR, rm: u32, shift_byte: u8) u32 {
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const shift_amt = @truncate(u5, shift_byte);
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const bit_count: u8 = @typeInfo(u32).Int.bits;
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var result: u32 = 0x0000_0000;
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if (shift_byte == 0 or shift_byte == bit_count) {
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// Actualy LSR #32
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cpsr.c.write(rm >> 31 & 1 == 1);
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} else if (shift_byte < bit_count) {
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// We can perform a well-defined shift
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const carry_bit = shift_amt - 1;
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cpsr.c.write(rm >> carry_bit & 1 == 1);
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result = rm >> shift_amt;
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} else {
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// All bits have been shifted out, including carry bit
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cpsr.c.write(false);
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}
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return result;
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}
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pub fn arithmetic_right(_: *CPSR, _: u32, _: u8) u32 {
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// @bitCast(u32, @bitCast(i32, r_val) >> @truncate(u5, amount))
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std.debug.panic("[BarrelShifter] implement arithmetic shift right", .{});
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}
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pub fn rotate_right(_: *CPSR, _: u32, _: u8) u32 {
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// std.math.rotr(u32, r_val, amount)
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std.debug.panic("[BarrelShifter] implement rotate right", .{});
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}
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