fix(rp2040): Fix CMP reg T1 carry flag (#31)

Author: Turro75

src/instructions.spec.ts

@@ -469,6 +469,18 @@ describe('Cortex-M0+ Instruction Set', () => {
     expect(registers.V).toEqual(false);
   });
 
+  it('should execute an `cmp r2, r0` instruction and not set any flags when r0=0xb71b0000 and r2=0x00b71b00', async () => {
+    await cpu.setPC(0x20000000);
+    await cpu.writeUint16(0x20000000, opcodeCMPregT1(r2, r0));
+    await cpu.setRegisters({ r0: 0xb71b0000, r2: 0x00b71b00 });
+    await cpu.singleStep();
+    const registers = await cpu.readRegisters();
+    expect(registers.N).toEqual(false);
+    expect(registers.Z).toEqual(false);
+    expect(registers.C).toEqual(false);
+    expect(registers.V).toEqual(false);
+  });
+
   it('should correctly set carry flag when executing `cmp r11, r3` instruction', async () => {
     await cpu.setPC(0x20000000);
     await cpu.writeUint16(0x20000000, opcodeCMPregT2(r11, r3));

src/rp2040.ts

@@ -1028,10 +1028,10 @@ export class RP2040 {
     else if (opcode >> 6 === 0b0100001010) {
       const Rm = (opcode >> 3) & 0x7;
       const Rn = opcode & 0x7;
-      const leftValue = this.registers[Rn] | 0;
-      const rightValue = this.registers[Rm] | 0;
-      const result = (leftValue - rightValue) | 0;
-      this.N = leftValue < rightValue;
+      const leftValue = this.registers[Rn];
+      const rightValue = this.registers[Rm];
+      const result = ((leftValue | 0) - (rightValue | 0)) | 0;
+      this.N = result < 0;
       this.Z = leftValue === rightValue;
       this.C = leftValue >= rightValue;
       this.V =