paoda
/
zba
1
0
Fork 0
Code Issues Pull Requests Projects Releases Wiki Activity

chore(cpu): refactor the barrel shifter once again

This commit is contained in:
Rekai Nyangadzayi Musuka 2022-10-21 05:11:59 -03:00
parent 43d011538e
commit 5d0bc1b335
2 changed files with 110 additions and 66 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

155
src/cpu/arm/barrel_shifter.zig
View File

@ -3,82 +3,142 @@ const std = @import("std");
const Arm7tdmi = @import("../../cpu.zig").Arm7tdmi;
const CPSR = @import("../../cpu.zig").PSR;
pub fn exec(comptime S: bool, cpu: *Arm7tdmi, opcode: u32) u32 {
var shift_amt: u8 = undefined;
pub fn execute(comptime S: bool, cpu: *Arm7tdmi, opcode: u32) u32 {
var result: u32 = undefined;
if (opcode >> 4 & 1 == 1) {
shift_amt = @truncate(u8, cpu.r[opcode >> 8 & 0xF]);
result = registerShift(S, cpu, opcode);
} else {
shift_amt = @truncate(u8, opcode >> 7 & 0x1F);
result = immShift(S, cpu, opcode);
}
const rm = cpu.r[opcode & 0xF];
var value: u32 = undefined;
if (rm == 0xF) {
value = cpu.fakePC() + 4; // 12 ahead
} else {
value = cpu.r[opcode & 0xF];
}
return result;
}
fn registerShift(comptime S: bool, cpu: *Arm7tdmi, opcode: u32) u32 {
const rs_idx = opcode >> 8 & 0xF;
const rs = @truncate(u8, cpu.r[rs_idx]);
const rm_idx = opcode & 0xF;
const rm = if (rm_idx == 0xF) cpu.fakePC() + 4 else cpu.r[rm_idx];
return switch (@truncate(u2, opcode >> 5)) {
0b00 => logicalLeft(S, &cpu.cpsr, value, shift_amt),
0b01 => logicalRight(S, &cpu.cpsr, value, shift_amt),
0b10 => arithmeticRight(S, &cpu.cpsr, value, shift_amt),
0b11 => rotateRight(S, &cpu.cpsr, value, shift_amt),
0b00 => logicalLeft(S, &cpu.cpsr, rm, rs),
0b01 => logicalRight(S, &cpu.cpsr, rm, rs),
0b10 => arithmeticRight(S, &cpu.cpsr, rm, rs),
0b11 => rotateRight(S, &cpu.cpsr, rm, rs),
};
}
pub fn logicalLeft(comptime S: bool, cpsr: *CPSR, rm: u32, amount: u8) u32 {
const shift_amt = @truncate(u5, amount);
fn immShift(comptime S: bool, cpu: *Arm7tdmi, opcode: u32) u32 {
const amount = @truncate(u8, opcode >> 7 & 0x1F);
const rm_idx = opcode & 0xF;
const rm = if (rm_idx == 0xF) cpu.fakePC() + 4 else cpu.r[rm_idx];
var result: u32 = undefined;
if (amount == 0) {
switch (@truncate(u2, opcode >> 5)) {
0b00 => {
// LSL #0
result = rm;
},
0b01 => {
// LSR #0 aka LSR #32
if (S) cpu.cpsr.c.write(rm >> 31 & 1 == 1);
result = 0x0000_0000;
},
0b10 => {
// ASR #0 aka ASR #32
result = @bitCast(u32, @bitCast(i32, rm) >> 31);
if (S) cpu.cpsr.c.write(result >> 31 & 1 == 1);
},
0b11 => {
// ROR #0 aka RRX
const carry: u32 = @boolToInt(cpu.cpsr.c.read());
if (S) cpu.cpsr.c.write(rm & 1 == 1);
result = (carry << 31) | (rm >> 1);
},
}
} else {
switch (@truncate(u2, opcode >> 5)) {
0b00 => result = logicalLeft(S, &cpu.cpsr, rm, amount),
0b01 => result = logicalRight(S, &cpu.cpsr, rm, amount),
0b10 => result = arithmeticRight(S, &cpu.cpsr, rm, amount),
0b11 => result = rotateRight(S, &cpu.cpsr, rm, amount),
}
}
return result;
}
pub fn logicalLeft(comptime S: bool, cpsr: *CPSR, rm: u32, total_amount: u8) u32 {
const amount = @truncate(u5, total_amount);
const bit_count: u8 = @typeInfo(u32).Int.bits;
var result: u32 = 0x0000_0000;
if (amount < bit_count) {
if (total_amount < bit_count) {
// We can perform a well-defined shift here
result = rm << amount;
// FIXME: We assume cpu.r[rs] == 0 and imm_shift == 0 are equivalent
if (S and shift_amt != 0) {
const carry_bit = @truncate(u5, bit_count - shift_amt);
if (S) {
const carry_bit = @truncate(u5, bit_count - amount);
cpsr.c.write(rm >> carry_bit & 1 == 1);
}
result = rm << shift_amt;
} else if (amount == bit_count) {
// Shifted all bits out, carry bit is bit 0 of rm
if (S) cpsr.c.write(rm & 1 == 1);
} else {
// Shifted all bits out, carry bit has also been shifted out
if (S) cpsr.c.write(false);
if (S) {
if (total_amount == bit_count) {
// Shifted all bits out, carry bit is bit 0 of rm
cpsr.c.write(rm & 1 == 1);
} else {
cpsr.c.write(false);
}
}
}
return result;
}
pub fn logicalRight(comptime S: bool, cpsr: *CPSR, rm: u32, amount: u32) u32 {
const shift_amt = @truncate(u5, amount);
pub fn logicalRight(comptime S: bool, cpsr: *CPSR, rm: u32, total_amount: u32) u32 {
const amount = @truncate(u5, total_amount);
const bit_count: u8 = @typeInfo(u32).Int.bits;
var result: u32 = 0x0000_0000;
if (amount == 0 or amount == bit_count) {
// Actualy LSR #32
if (S) cpsr.c.write(rm >> 31 & 1 == 1);
} else if (amount < bit_count) {
if (total_amount < bit_count) {
// We can perform a well-defined shift
const carry_bit = shift_amt - 1;
if (S) cpsr.c.write(rm >> carry_bit & 1 == 1);
result = rm >> shift_amt;
result = rm >> amount;
if (S) cpsr.c.write(rm >> (amount - 1) & 1 == 1);
} else {
if (S) {
if (total_amount == bit_count) {
// LSR #32
cpsr.c.write(rm >> 31 & 1 == 1);
} else {
// All bits have been shifted out, including carry bit
if (S) cpsr.c.write(false);
cpsr.c.write(false);
}
}
}
return result;
}
pub fn arithmeticRight(comptime _: bool, _: *CPSR, _: u32, _: u8) u32 {
// @bitCast(u32, @bitCast(i32, r_val) >> @truncate(u5, amount))
pub fn arithmeticRight(comptime S: bool, cpsr: *CPSR, rm: u32, total_amount: u8) u32 {
const amount = @truncate(u5, total_amount);
const bit_count: u8 = @typeInfo(u32).Int.bits;
var result: u32 = 0x0000_0000;
if (total_amount < bit_count) {
result = @bitCast(u32, @bitCast(i32, rm) >> amount);
if (S) cpsr.c.write(rm >> (amount - 1) & 1 == 1);
} else {
if (S) {
// ASR #32 and ASR #>32 have the same result
result = @bitCast(u32, @bitCast(i32, rm) >> 31);
cpsr.c.write(result >> 31 & 1 == 1);
}
}
std.debug.panic("[BarrelShifter] implement arithmetic shift right", .{});
}
@ -91,12 +151,3 @@ pub fn rotateRight(comptime S: bool, cpsr: *CPSR, rm: u32, amount: u8) u32 {
return result;
}
pub fn rotateRightExtended(comptime S: bool, cpsr: *CPSR, rm: u32) u32 {
if (!S) std.debug.panic("[BarrelShifter] Turns out I don't know how RRX works", .{});
const carry: u32 = @boolToInt(cpsr.c.read());
cpsr.c.write(rm & 1 == 1);
return (carry << 31) | (rm >> 1);
}

19
src/cpu/arm/data_processing.zig
View File

@ -1,6 +1,6 @@
const std = @import("std");
const BarrelShifter = @import("barrel_shifter.zig");
const shifter = @import("barrel_shifter.zig");
const Bus = @import("../../Bus.zig");
const Arm7tdmi = @import("../../cpu.zig").Arm7tdmi;
const InstrFn = @import("../../cpu.zig").ArmInstrFn;
@ -11,21 +11,14 @@ pub fn dataProcessing(comptime I: bool, comptime S: bool, comptime instrKind: u4
const rd = opcode >> 12 & 0xF;
const rn = opcode >> 16 & 0xF;
if (S and rd == 0xF) std.debug.panic("[CPU] Data Processing Instruction w/ S set and Rd == 15", .{});
var op1: u32 = undefined;
if (rn == 0xF) {
op1 = cpu.fakePC();
} else {
op1 = cpu.r[rn];
}
const op1 = if (rn == 0xF) cpu.fakePC() else cpu.r[rn];
var op2: u32 = undefined;
if (I) {
const amount = @truncate(u8, (opcode >> 8 & 0xF) << 1);
op2 = BarrelShifter.rotateRight(S, &cpu.cpsr, opcode & 0xFF, amount);
op2 = shifter.rotateRight(S, &cpu.cpsr, opcode & 0xFF, amount);
} else {
op2 = BarrelShifter.exec(S, cpu, opcode);
op2 = shifter.execute(S, cpu, opcode);
}
switch (instrKind) {
@ -77,7 +70,7 @@ pub fn dataProcessing(comptime I: bool, comptime S: bool, comptime instrKind: u4
cpu.cpsr.n.write(result >> 31 & 1 == 1);
cpu.cpsr.z.write(result == 0);
// Barrel Shifter should always calc CPSR C in TST
if (!S) _ = BarrelShifter.exec(true, cpu, opcode);
if (!S) _ = shifter.execute(true, cpu, opcode);
},
0x9 => {
// TEQ
@ -86,7 +79,7 @@ pub fn dataProcessing(comptime I: bool, comptime S: bool, comptime instrKind: u4
cpu.cpsr.n.write(result >> 31 & 1 == 1);
cpu.cpsr.z.write(result == 0);
// Barrel Shifter should always calc CPSR C in TEQ
if (!S) _ = BarrelShifter.exec(true, cpu, opcode);
if (!S) _ = shifter.execute(true, cpu, opcode);
},
0xD => {
// MOV