Add RP references with first letter of register name

src/assembler.js

@@ -411,12 +411,10 @@ class Assembler {
   // Expect arg to be a register pair and return its encoding.
   _argRP(sl, arg) {
     switch (arg.toLowerCase()) {
-      case 'bc': return 0b00;
-      case 'de': return 0b01;
-      case 'hl': return 0b10;
-      case 'sp':
-      case 'psw':
-        return 0b11;
+      case 'bc': case 'b': return 0b00;
+      case 'de': case 'd': return 0b01;
+      case 'hl': case 'h': return 0b10;
+      case 'sp': case 'psw': return 0b11;
       default:
         this._assemblyError(sl.pos, `invalid register pair ${arg}`);
     }

test/test.js

@@ -487,6 +487,24 @@ There:  mvi a, 30
     assert.equal(state.e, 0x34);
   });
 
+  it('rp-single-char-ref', () => {
+    // Test that referring to register pairs by the name of the first register
+    // in the pair works OK.
+    let [state, mem] = runProg(`
+      lxi h, 1234h
+      lxi d, 4567h
+      lxi b, abcdh
+      hlt
+      `);
+    assert.ok(state.halted);
+    assert.equal(state.h, 0x12);
+    assert.equal(state.l, 0x34);
+    assert.equal(state.d, 0x45);
+    assert.equal(state.e, 0x67);
+    assert.equal(state.b, 0xab);
+    assert.equal(state.c, 0xcd);
+  });
+
   it('inx', () => {
     let [state, mem] = runProg(`
       lxi hl, 12ffh
@@ -547,13 +565,10 @@ loop:
 `);
 
     assert.ok(state.halted);
-    console.log(mem);
-
     assert.equal(mem[labelToAddr.get('TargetArray')], 0x11);
     assert.equal(mem[labelToAddr.get('TargetArray') + 1], 0x22);
     assert.equal(mem[labelToAddr.get('TargetArray') + 2], 0x33);
     assert.equal(mem[labelToAddr.get('TargetArray') + 3], 0x44);
     assert.equal(mem[labelToAddr.get('TargetArray') + 4], 0x55);
   });
-
 });