From 9860294329a9c64b04f7c07dc17598d0d8215f6a Mon Sep 17 00:00:00 2001
From: Rekai Musuka <rekai@musuka.dev>
Date: Wed, 12 Jan 2022 06:54:22 -0400
Subject: [PATCH] fix(cpu): make Data Processing instructions r15-aware

---
 src/cpu/barrel_shifter.zig  | 22 ++++++++++++++--------
 src/cpu/data_processing.zig | 29 +++++++++++++++++------------
 2 files changed, 31 insertions(+), 20 deletions(-)

diff --git a/src/cpu/barrel_shifter.zig b/src/cpu/barrel_shifter.zig
index 4ab804f..31052d8 100644
--- a/src/cpu/barrel_shifter.zig
+++ b/src/cpu/barrel_shifter.zig
@@ -12,21 +12,27 @@ pub fn exec(comptime S: bool, cpu: *Arm7tdmi, opcode: u32) u32 {
     }
 
     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];
+    }
 
     if (S) {
         return switch (@truncate(u2, opcode >> 5)) {
-            0b00 => logical_left(&cpu.cpsr, rm, shift_amt),
-            0b01 => logical_right(&cpu.cpsr, rm, shift_amt),
-            0b10 => arithmetic_right(&cpu.cpsr, rm, shift_amt),
-            0b11 => rotate_right(&cpu.cpsr, rm, shift_amt),
+            0b00 => logical_left(&cpu.cpsr, value, shift_amt),
+            0b01 => logical_right(&cpu.cpsr, value, shift_amt),
+            0b10 => arithmetic_right(&cpu.cpsr, value, shift_amt),
+            0b11 => rotate_right(&cpu.cpsr, value, shift_amt),
         };
     } else {
         var dummy = CPSR{ .raw = 0x0000_0000 };
         return switch (@truncate(u2, opcode >> 5)) {
-            0b00 => logical_left(&dummy, rm, shift_amt),
-            0b01 => logical_right(&dummy, rm, shift_amt),
-            0b10 => arithmetic_right(&dummy, rm, shift_amt),
-            0b11 => rotate_right(&dummy, rm, shift_amt),
+            0b00 => logical_left(&dummy, value, shift_amt),
+            0b01 => logical_right(&dummy, value, shift_amt),
+            0b10 => arithmetic_right(&dummy, value, shift_amt),
+            0b11 => rotate_right(&dummy, value, shift_amt),
         };
     }
 }
diff --git a/src/cpu/data_processing.zig b/src/cpu/data_processing.zig
index 12a045d..9b96690 100644
--- a/src/cpu/data_processing.zig
+++ b/src/cpu/data_processing.zig
@@ -9,24 +9,29 @@ pub fn dataProcessing(comptime I: bool, comptime S: bool, comptime instrKind: u4
     return struct {
         fn inner(cpu: *Arm7tdmi, _: *Bus, opcode: u32) void {
             const rd = opcode >> 12 & 0xF;
-            const op1 = opcode >> 16 & 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];
+            }
 
             var op2: u32 = undefined;
             if (I) {
                 op2 = std.math.rotr(u32, opcode & 0xFF, (opcode >> 8 & 0xF) << 1);
             } else {
-                if (S and rd == 0xF) {
-                    std.debug.panic("[CPU] Data Processing Instruction w/ S set and Rd == 15", .{});
-                } else {
-                    op2 = BarrelShifter.exec(S, cpu, opcode);
-                }
+                op2 = BarrelShifter.exec(S, cpu, opcode);
             }
 
             switch (instrKind) {
                 0x4 => {
                     // ADD
                     var result: u32 = undefined;
-                    const didOverflow = @addWithOverflow(u32, cpu.r[op1], op2, &result);
+                    const didOverflow = @addWithOverflow(u32, op1, op2, &result);
                     cpu.r[rd] = result;
 
                     if (S and rd != 0xF) {
@@ -38,7 +43,7 @@ pub fn dataProcessing(comptime I: bool, comptime S: bool, comptime instrKind: u4
                 },
                 0x8 => {
                     // TST
-                    const result = cpu.r[op1] & op2;
+                    const result = op1 & op2;
 
                     cpu.cpsr.n.write(result >> 31 & 1 == 1);
                     cpu.cpsr.z.write(result == 0);
@@ -47,7 +52,7 @@ pub fn dataProcessing(comptime I: bool, comptime S: bool, comptime instrKind: u4
                 },
                 0x9 => {
                     // TEQ
-                    const result = cpu.r[op1] ^ op2;
+                    const result = op1 ^ op2;
 
                     cpu.cpsr.n.write(result >> 31 & 1 == 1);
                     cpu.cpsr.z.write(result == 0);
@@ -66,16 +71,16 @@ pub fn dataProcessing(comptime I: bool, comptime S: bool, comptime instrKind: u4
                 },
                 0xA => {
                     // CMP
-                    const result = cpu.r[op1] -% op2;
+                    const result = op1 -% op2;
 
                     cpu.cpsr.n.write(result >> 31 & 1 == 1);
                     cpu.cpsr.z.write(result == 0);
-                    cpu.cpsr.c.write(op2 <= cpu.r[op1]);
+                    cpu.cpsr.c.write(op2 <= op1);
                     cpu.cpsr.v.write(((op1 ^ result) & (~op2 ^ result)) >> 31 & 1 == 1);
                 },
                 0xC => {
                     // ORR
-                    const result = cpu.r[op1] | op2;
+                    const result = op1 | op2;
                     cpu.r[rd] = result;
 
                     if (S and rd != 0xF) {