2 files changed, 50 insertions, 9 deletions
diff --git a/src/instruction.spec.ts b/src/instruction.spec.ts
index 8d3ac3d..0cfaf03 100644
--- a/src/instruction.spec.ts
+++ b/src/instruction.spec.ts
@@ -58,6 +58,39 @@ describe('avrInstruction', () => {
     expect(cpu.data[28]).toEqual(0xff);
   });
 
+  it('should execute `LD r1, X` instruction', () => {
+    loadProgram('1c90');
+    cpu.data[0xc0] = 0x15;
+    cpu.data[26] = 0xc0; // X <- 0xc0
+    avrInstruction(cpu);
+    expect(cpu.pc).toEqual(1);
+    expect(cpu.cycles).toEqual(1);
+    expect(cpu.data[1]).toEqual(0x15);
+    expect(cpu.data[26]).toEqual(0xc0); // verify that X was unchanged
+  });
+
+  it('should execute `LD r17, X+` instruction', () => {
+    loadProgram('1d91');
+    cpu.data[0xc0] = 0x15;
+    cpu.data[26] = 0xc0; // X <- 0x9a
+    avrInstruction(cpu);
+    expect(cpu.pc).toEqual(1);
+    expect(cpu.cycles).toEqual(2);
+    expect(cpu.data[0xc0]).toEqual(0x15);
+    expect(cpu.data[26]).toEqual(0xc1); // verify that X was incremented
+  });
+
+  it('should execute `LD r1, -X` instruction', () => {
+    loadProgram('1e90');
+    cpu.data[0x98] = 0x22;
+    cpu.data[26] = 0x99; // X <- 0x99
+    avrInstruction(cpu);
+    expect(cpu.pc).toEqual(1);
+    expect(cpu.cycles).toEqual(3);
+    expect(cpu.data[1]).toEqual(0x22);
+    expect(cpu.data[26]).toEqual(0x98); // verify that X was decremented
+  });
+
   it('should execute `RJMP 2` instruction', () => {
     loadProgram('01c0');
     avrInstruction(cpu);
@@ -87,7 +120,7 @@ describe('avrInstruction', () => {
     expect(cpu.data[26]).toEqual(0x9b); // verify that X was incremented
   });
 
-  it('should execute `ST X-, r17` instruction', () => {
+  it('should execute `ST -X, r17` instruction', () => {
     loadProgram('1e93');
     cpu.data[17] = 0x88; // r17 <- 0x88
     cpu.data[26] = 0x99; // X <- 0x99
@@ -95,6 +128,6 @@ describe('avrInstruction', () => {
     expect(cpu.pc).toEqual(1);
     expect(cpu.cycles).toEqual(2);
     expect(cpu.data[0x98]).toEqual(0x88);
-    expect(cpu.data[26]).toEqual(0x98); // verify that X was unchanged
+    expect(cpu.data[26]).toEqual(0x98); // verify that X was decremented
   });
 });
diff --git a/src/instruction.ts b/src/instruction.ts
index 1d016aa..8e52a80 100644
--- a/src/instruction.ts
+++ b/src/instruction.ts
@@ -204,17 +204,23 @@ export function avrInstruction(cpu: ICPU) {
 
   /* LDX, 1001 000d dddd 1100 */
   if ((opcode & 0xfe0f) === 0x900c) {
-    /* not implemented */
+    cpu.data[(opcode & 0x1f0) >> 4] = cpu.readData(cpu.dataView.getUint16(26, true));
   }
 
   /* LDX, 1001 000d dddd 1101 */
   if ((opcode & 0xfe0f) === 0x900d) {
-    /* not implemented */
+    const x = cpu.dataView.getUint16(26, true);
+    cpu.data[(opcode & 0x1f0) >> 4] = x;
+    cpu.dataView.setUint16(26, x + 1, true);
+    cpu.cycles++;
   }
 
   /* LDX, 1001 000d dddd 1110 */
   if ((opcode & 0xfe0f) === 0x900e) {
-    /* not implemented */
+    const x = cpu.dataView.getUint16(26, true) - 1;
+    cpu.dataView.setUint16(26, x, true);
+    cpu.data[(opcode & 0x1f0) >> 4] = cpu.readData(x);
+    cpu.cycles += 2;
   }
 
   /* LDY, 1000 000d dddd 1000 */
@@ -430,15 +436,17 @@ export function avrInstruction(cpu: ICPU) {
 
   /* STX, 1001 001r rrrr 1101 */
   if ((opcode & 0xfe0f) === 0x920d) {
-    cpu.writeData(cpu.dataView.getUint16(26, true), cpu.data[(opcode & 0x1f0) >> 4]);
-    cpu.dataView.setUint16(26, cpu.dataView.getUint16(26, true) + 1, true);
+    const x = cpu.dataView.getUint16(26, true);
+    cpu.writeData(x, cpu.data[(opcode & 0x1f0) >> 4]);
+    cpu.dataView.setUint16(26, x + 1, true);
   }
 
   /* STX, 1001 001r rrrr 1110 */
   if ((opcode & 0xfe0f) === 0x920e) {
     const i = cpu.data[(opcode & 0x1f0) >> 4];
-    cpu.dataView.setUint16(26, cpu.dataView.getUint16(26, true) - 1, true);
-    cpu.writeData(cpu.dataView.getUint16(26, true), i);
+    const x = cpu.dataView.getUint16(26, true) - 1;
+    cpu.dataView.setUint16(26, x, true);
+    cpu.writeData(x, i);
     cpu.cycles++;
   }