Add support for the 'dw' directive

TODO

@@ -8,3 +8,5 @@ Links:
 * Online assembler with "live" assmelby:
   https://svofski.github.io/pretty-8080-assembler/
 * Online assembler: https://www.asm80.com/onepage/asm8080.html 
+* Intel 8080-8085 assembly manual:
+  https://www.tramm.li/i8080/Intel%208080-8085%20Assembly%20Language%20Programming%201977%20Intel.pdf

src/assembler.js

@@ -28,6 +28,9 @@ class Assembler {
     this.tracing = false;
   }
 
+  // Assembles the given source lines. Returns [memory, labelToAddr].
+  // memory is the assembled memory array. labelToAddr is a Map object mapping
+  // labels to addresses in memory.
   assemble(sourceLines) {
     this._assembleInstructions(sourceLines);
     this._applyFixups();
@@ -189,6 +192,17 @@ class Assembler {
         // Pseudo-instruction that simply assigns its immediate args to memory.
         return sl.args.map((arg) => {return this._argImm(sl, arg);});
       }
+      case 'dw': {
+        // Pseudo-instruction that treats immediate args as 2-byte words, and
+        // assigns them to memory in little-endian.
+        let enc = [];
+        for (let arg of sl.args) {
+          let argEnc = this._argImm(sl, arg);
+          enc.push(argEnc & 0xFF);
+          enc.push((argEnc >> 8) & 0xFF);
+        }
+        return enc;
+      }
       case 'dcr': {
         this._expectArgsCount(sl, 1);
         let r = this._argR(sl, sl.args[0]);

test/test.js

@@ -229,6 +229,22 @@ describe('sim', () => {
     assert.equal(state.a, 33);
   });
 
+  it('array-dw', () => {
+    let [state, mem, labelToAddr] = runProg(`
+      hlt
+
+    myArray:
+      dw 2030h, 5060h
+    `);
+
+    assert.ok(state.halted);
+    let arrAddr = labelToAddr.get('myArray');
+    assert.equal(mem[arrAddr], 0x30);
+    assert.equal(mem[arrAddr+1], 0x20);
+    assert.equal(mem[arrAddr+2], 0x60);
+    assert.equal(mem[arrAddr+3], 0x50);
+  });
+
   it('mult', () => {
     let [state, mem] = runProg(`
             mvi b, 44