2 files changed, 84 insertions, 1 deletions
diff --git a/src/vm/instruction.js b/src/vm/instruction.js
index c9048e1..146a88b 100644
--- a/src/vm/instruction.js
+++ b/src/vm/instruction.js
@@ -37,3 +37,18 @@ exports.ADD = function(core, pc, ins) {
 
 	return [pc + 1];
 }
+
+
+exports.SUB = function(core, pc, ins) {
+	if (ins.a.mode === AddrMode.Immediate) {
+		const dst = core.getValue(pc, ins.b);
+		dst.b.value -= ins.a.value;
+	} else {
+		const src = core.getValue(pc, ins.a);
+		const dst = core.getValue(pc, ins.b);
+		dst.a.value -= src.a.value;
+		dst.b.value -= src.b.value;
+	}
+
+	return [pc + 1];
+}
diff --git a/src/vm/instruction.test.js b/src/vm/instruction.test.js
index fd402e4..7bd2068 100644
--- a/src/vm/instruction.test.js
+++ b/src/vm/instruction.test.js
@@ -1,7 +1,7 @@
 'use strict';
 
 const { Core } = require('./core.js');
-const { DAT, MOV, ADD } = require('./instruction.js');
+const { DAT, MOV, ADD, SUB } = require('./instruction.js');
 
 
 const CORESIZE = 8000;
@@ -175,3 +175,71 @@ test('ADD correctly adds B-field', () => {
 		b: { mode: 'immediate', value: 5 },
 	});
 });
+
+
+test('SUB correctly subtracts two full instructions', () => {
+	const core = new Core(CORESIZE);
+	const pc = 20;
+	core.data[pc] = {
+		opcode: 'SUB',
+		a: { mode: 'direct', value: 1 },
+		b: { mode: 'direct', value: 2 },
+	};
+	core.data[pc+1] = {
+		opcode: 'DAT',
+		a: { mode: 'immediate', value: 10 },
+		b: { mode: 'immediate', value: 20 },
+	};
+	core.data[pc+2] = {
+		opcode: 'DAT',
+		a: { mode: 'immediate', value: 3 },
+		b: { mode: 'immediate', value: 4 },
+	};
+	const ins = core.data[pc];
+
+	expect(SUB(core, pc, ins)).toEqual([pc+1]);
+	expect(core.data[pc+1]).toEqual({
+		opcode: 'DAT',
+		a: { mode: 'immediate', value: 10 },
+		b: { mode: 'immediate', value: 20 },
+	});
+	expect(core.data[pc+2]).toEqual({
+		opcode: 'DAT',
+		a: { mode: 'immediate', value: -7 },
+		b: { mode: 'immediate', value: -16 },
+	});
+});
+
+
+test('SUB correctly subtracts B-field', () => {
+	const core = new Core(CORESIZE);
+	const pc = 20;
+	core.data[pc] = {
+		opcode: 'SUB',
+		a: { mode: 'immediate', value: 1 },
+		b: { mode: 'direct', value: 2 },
+	};
+	core.data[pc+1] = {
+		opcode: 'DAT',
+		a: { mode: 'immediate', value: 10 },
+		b: { mode: 'immediate', value: 20 },
+	};
+	core.data[pc+2] = {
+		opcode: 'DAT',
+		a: { mode: 'immediate', value: 3 },
+		b: { mode: 'immediate', value: 4 },
+	};
+	const ins = core.data[pc];
+
+	expect(SUB(core, pc, ins)).toEqual([pc+1]);
+	expect(core.data[pc+1]).toEqual({
+		opcode: 'DAT',
+		a: { mode: 'immediate', value: 10 },
+		b: { mode: 'immediate', value: 20 },
+	});
+	expect(core.data[pc+2]).toEqual({
+		opcode: 'DAT',
+		a: { mode: 'immediate', value: 3 },
+		b: { mode: 'immediate', value: 3 },
+	});
+});