Add constants to assembler

assembler.js

@@ -34,9 +34,10 @@ function decodeInstructions(str) {
   machineCode[0] = POINTER_TO_START_OF_DISPLAY_MEM;
 
   let labels = {};
+  let constants = {};
   let IP = INITIAL_IP_ADDRESS;
   for (let i = 0; i < lines.length; i++) {
-    dbg(1, '');
+    dbg(2, '');
     dbgGroup(1, `Input line ${i}, IP ${num2hex(IP)}`);
     dbg(3, `> ${lines[i]}`);
     let line = stripWhitespaceFromEnds(stripComments(lines[i]));
@@ -49,11 +50,14 @@ function decodeInstructions(str) {
       continue;
     }
 
-    // Handle labels -- anchors
+
+    // HANDLE OPS
+
+    // Handle label definitions
     if (line.startsWith('@')) {
       // TODO: validate label
       // validateLabel(line);
-      label = line.substring(1); // strip '@'
+      let label = line.substring(1); // strip '@'
 
       if (label in labels) {
         labels[label].pointsToByte = IP;
@@ -72,21 +76,36 @@ function decodeInstructions(str) {
 
     let op_arg_array = line.split(" "); // split line into an array of [op, arg]
     let opName = op_arg_array[0].toLowerCase();
-    let addressingMode = 'direct'; // Must be "direct" or "indirect"
     let arg_str = op_arg_array[1];
     let arg_num = null;
+    let addressingMode = 'direct'; // Must be "direct" or "indirect"
 
+    // Handle constant definitions
+    if (opName.startsWith('=')) {
+      // TODO: validate constant
+      let constantName = opName.substring(1); // strip '>'
+      let constantValue = arg_str;
+      constants[constantName] = constantValue;
+      dbg(2, `constants:`);
+      dbg(2, constants);
+      continue;
+    }
+
+    // Handle mnemonics without arguments (eg END) ...
     if (typeof arg_str === 'undefined') {
-      // Handle mnemonics without arguments (eg END) ...
       if (mnemonicsWithOptionalArgs.indexOf(opName) < 0) {
         console.error(`Missing opcode: ${line}`);
         throw new Error("Missing opcode");
       }
       arg_num = 0;
+
+
+    // HANDLE ARGUMENTS
+
+    // Handle references to labels
     } else if (arg_str.startsWith('@')) {
-      // Handle mnemonics with pointers to labels
       // TODO: validate label // validateLabel(line);
-      label = arg_str.substring(1); // strip '@'
+      let label = arg_str.substring(1); // strip '@'
       arg_num = 0;
 
       if (label in labels) {
@@ -100,8 +119,24 @@ function decodeInstructions(str) {
       }
       dbg(2, `pointsToByte: ${labels[label].pointsToByte}`);
       dbg(2, `bytesToReplace: ${labels[label].bytesToReplace}`);
+
+    // Handle references to constants
+    } else if (arg_str.startsWith('=')) {
+      arg_str = arg_str.substring(1) // strip '>'
+      dbg(2, `argument references '${arg_str}'`);
+      arg_str = constants[arg_str];
+      dbg(2, `arg_str from '${arg_str}`);
+    
+    // Handle references to constants in indirect mode
+    } else if (arg_str.startsWith("(=")) {
+      addressingMode = "indirect";
+      arg_str = arg_str.replace("(=", "");
+      arg_str = arg_str.replace(")", "");
+      dbg(2, `INDY - argument references '${arg_str}'`);
+      arg_str = constants[arg_str];
+
+    // Handle indirect expressions
     } else if (arg_str.startsWith("(")) {
-      // Handle indirect expressions
       addressingMode = "indirect";
       arg_str = arg_str.replace("(", "");
       arg_str = arg_str.replace(")", "");
@@ -119,7 +154,7 @@ function decodeInstructions(str) {
       }
     }
 
-    // Decode!
+    // DECODE!
     op = mnemonics2opcodes[opName][addressingMode];
 
     machineCode.push(op);
@@ -130,11 +165,11 @@ function decodeInstructions(str) {
   };
 
   dbg(1, '');
-  dbgGroup(1, 'Memory before filling in label pointers');
+  dbgGroup(1, 'Memory before filling in label constants');
   dbgExec(1, () => logMemory(machineCode));
-  dbgGroupEnd(1, 'Memory before filling in label pointers');
+  dbgGroupEnd(1, 'Memory before filling in label constants');
 
-  // Backfill label pointers
+  // Backfill label references
   for (let k of Object.keys(labels)) {
     dbgGroup(2, `@${k}`);
     let label = labels[k];

readme.md

@@ -10,12 +10,14 @@
 
 ### Features
 
-- [ ] Make $0F a NOP
+- [ ] Assembler: pad to 256 bytes, OR, to start, always add an END
+- [x] Add constants to assembler
+- [ ] Keypad
+- [ ] Add single-stepping 
+- [x] Make $0F a NOP
 - [ ] In assembler.js: validateLabel()
 - [ ] In assemble.js: print better output to stdout
 - [ ] Add a function for logging just a specific range of memory
-- [ ] Add single-stepping 
-- [ ] Add variables to assembler
 
 ### Under-the-hood improvements
 
@@ -104,5 +106,10 @@ Assemble and run, with debug output:
                     ; the label will be replaced with
                     ; the address of the label
 
+    =foo $FF        ; define a constant
+                    ; (must be defined before it is referenced)
+    
+    ADD =foo        ; use a constant as an operand
+
 - Hexadecimal numbers are preceded by a `$`
 - Whitespace is ignored

test-programs/constants.asm

@@ -0,0 +1,7 @@
+;; Test constants
+=foo $01
+=bar $02
+ADD =foo
+STO =bar
+STO (=bar)
+END