From c64f09781021d2771cf85e1378f0a9753d54bc21 Mon Sep 17 00:00:00 2001
From: Anachronaut <75824887+RealBusinessAccount@users.noreply.github.com>
Date: Wed, 30 Oct 2024 16:06:33 -0400
Subject: [PATCH] Added bit shift instructions.

SHL and SHR treat A and B as a 16 bit circular shift register and rotates them either left or right.
---
 Source/Assembler/assembly.c    |  2 ++
 Source/Emulator/cpu.c          | 16 ++++++++++++++++
 SplitBit Programming Manual.md |  4 +++-
 3 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/Source/Assembler/assembly.c b/Source/Assembler/assembly.c
index b08c6d6..769c2cd 100644
--- a/Source/Assembler/assembly.c
+++ b/Source/Assembler/assembly.c
@@ -22,6 +22,8 @@ Instruction instruction_set[] = {
     {0x06, "XOR"},
     {0x07, "NOTA"},
     {0x08, "NOTB"},
+    {0x09, "SHL"},
+    {0x0A, "SHR"},
     // Branch Operations:
     {0x10, "BRI"},
     {0x11, "BRQ"},
diff --git a/Source/Emulator/cpu.c b/Source/Emulator/cpu.c
index 389048e..9a877c0 100644
--- a/Source/Emulator/cpu.c
+++ b/Source/Emulator/cpu.c
@@ -6,6 +6,8 @@
 #include "cpu.h"
 #include "io.h"
 
+uint16_t shiftRegister;
+
 void initializeCPU(CPURegisters *cpu, uint8_t *programMemory, uint8_t *dataMemory) {
     cpu->A = 0;
     cpu->B = 0;
@@ -80,6 +82,20 @@ uint8_t executeOperation(uint8_t Instruction, CPURegisters *cpu) {
             // NOTB - not B -> Q
             cpu->Q = ~cpu->B;
         break;
+        case 0x09:
+            // SHL - Shift AB left.
+            shiftRegister = ((uint16_t)cpu->A << 8) | cpu->B;
+            shiftRegister = (shiftRegister << 1) | (shiftRegister >> 15);
+            cpu->A = shiftRegister >> 8;
+            cpu->B = shiftRegister & 0xFF;
+        break;
+        case 0x0A:
+            // SHR - Shift AB right.
+            shiftRegister = ((uint16_t)cpu->A << 8) | cpu->B;
+            shiftRegister = (shiftRegister >> 1) | (shiftRegister << 15);
+            cpu->A = shiftRegister >> 8;
+            cpu->B = shiftRegister & 0xFF;
+        break;
         //
         // 1x - Branch Operations:
         //
diff --git a/SplitBit Programming Manual.md b/SplitBit Programming Manual.md
index d10c46e..cf3bd5f 100644
--- a/SplitBit Programming Manual.md	
+++ b/SplitBit Programming Manual.md	
@@ -16,7 +16,7 @@ It has six registers:
 
 ## List of Instructions:
 
-### Arithmetic and Logic Operations: 9 Instructions
+### Arithmetic and Logic Operations: 11 Instructions
 Hex Code | Mnemonic | Description
 -- | -- | --
 00 | ADD |  Adds A, B, and the Carry Flag, the result is stored in Q.
@@ -28,6 +28,8 @@ Hex Code | Mnemonic | Description
 06 | XOR | Bitwise xor of A and B, the result is stored in Q.
 07 | NOTA | Bitwise inversion of A, the result is stored in Q.
 08 | NOTB | Bitwise inversion of B, the result is stored in Q.
+09 | SHL | A and B form a circular shift register. Rotate this register left.
+0A | SHR | A and B form a circular shift register. Rotate this register right.
 
 ### Branch Operations: 7 Instructions
 Hex Code | Mnemonic | Description