2 files changed, 60 insertions, 0 deletions
diff --git a/src/vm/instruction.js b/src/vm/instruction.js
index f323f84..c235b9a 100644
--- a/src/vm/instruction.js
+++ b/src/vm/instruction.js
@@ -142,3 +142,23 @@ exports.JMN = function(core, pc, ins) {
 		return [core.normalize(pc, 1)];
 	}
 }
+
+
+exports.DJN = function(core, pc, ins) {
+	let test;
+
+	if (ins.b.mode === AddrMode.Immediate) {
+		ins.b.value -= 1;
+		test = (ins.b.value !== 0);
+	} else {
+		const src = core.getValue(pc, ins.b);
+		src.b.value -= 1;
+		test = (src.b.value !== 0);
+	}
+
+	if (test) {
+		return [core.getLocation(pc, ins.a)];
+	} else {
+		return [core.normalize(pc, 1)];
+	}
+}
diff --git a/src/vm/instruction.test.js b/src/vm/instruction.test.js
index 2880da7..82d9819 100644
--- a/src/vm/instruction.test.js
+++ b/src/vm/instruction.test.js
@@ -466,3 +466,43 @@ test('JMN correctly jumps in immediate mode', () => {
 	expect(JMN(core, pc, ins)).toEqual([CORESIZE-1]);
 });
 
+
+test('DJN correctly jumps in non-immediate mode', () => {
+	const core = new Core(CORESIZE);
+	const pc = 2;
+	core.data[pc] = {
+		opcode: 'DJN',
+		a: { mode: 'direct', value: -3 },
+		b: { mode: 'direct', value: 1 },
+	};
+	core.data[pc+1] = {
+		opcode: 'DJN',
+		a: { mode: 'direct', value: -3 },
+		b: { mode: 'direct', value: 1 },
+	};
+	const ins = core.data[pc];
+
+	expect(DJN(core, pc, ins)).toEqual([pc+1]);
+	expect(core.data[pc+1].b.value).toBe(0);
+	expect(DJN(core, pc, ins)).toEqual([CORESIZE-1]);
+	expect(core.data[pc+1].b.value).toBe(-1);
+});
+
+
+test('DJN correctly jumps in immediate mode', () => {
+	const core = new Core(CORESIZE);
+	const pc = 2;
+	core.data[pc] = {
+		opcode: 'DJN',
+		a: { mode: 'direct', value: -3 },
+		b: { mode: 'immediate', value: 1 },
+	};
+	const ins = core.data[pc];
+
+	expect(DJN(core, pc, ins)).toEqual([pc+1]);
+	expect(core.data[pc].b.value).toBe(0);
+	expect(DJN(core, pc, ins)).toEqual([CORESIZE-1]);
+	expect(core.data[pc].b.value).toBe(-1);
+});
+
+