From 4e01a0943cc070b92a1305a215ed4fdb3b5a2607 Mon Sep 17 00:00:00 2001
From: n loewen <nick@nloewen.com>
Date: Tue, 15 Aug 2023 21:29:28 +0100
Subject: [PATCH] Feature: Add `*ADDR` assembly operation (`*ADDR`, used as an
 operand, is replaced with the memory address where the current line will be
 stored after assembly)

---
 assembler.js                                       |  8 ++++++++
 notes/todo.md                                      |  1 +
 readme.md                                          |  3 +++
 test-programs/referencing-addr-during-assembly.asm | 11 +++++++++++
 4 files changed, 23 insertions(+)
 create mode 100644 test-programs/referencing-addr-during-assembly.asm

diff --git a/assembler.js b/assembler.js
index 96a3893..aac84df 100644
--- a/assembler.js
+++ b/assembler.js
@@ -123,12 +123,19 @@ function decodeInstructions(line) {
       }
       dbg(2, `pointsToByte: ${labels[label].pointsToByte}`);
       dbg(2, `bytesToReplace: ${labels[label].bytesToReplace}`);
+    
+    // Handle references to the Instruction Pointer
+    } else if (arg_str.toLowerCase() === '*addr') {
+      dbg(2, `operand references current address`);
+      arg_num = IP;
+      dbg(2, `arg_num: ${num2hex(arg_num)}`);
 
     // Handle references to constants
     } else if (arg_str.startsWith('=')) {
       // FIXME - a quick hack to get around problems caused by another use of lower-casing to sanitize input:
       arg_str = arg_str.substring(1).toLowerCase(); // strip '>'
       dbg(2, `operand references '${arg_str}'`);
+      // TODO: support *ADDR
       arg_str = constants[arg_str];
       dbg(2, `arg_str from '${arg_str}`);
     
@@ -140,6 +147,7 @@ function decodeInstructions(line) {
       // FIXME - a quick hack to get around problems caused by another use of lower-casing to sanitize input:
       arg_str = arg_str.toLowerCase();
       dbg(2, `INDY - operand references '${arg_str}'`);
+      // TODO: support *ADDR
       arg_str = constants[arg_str];
 
     // Handle indirect expressions
diff --git a/notes/todo.md b/notes/todo.md
index 92fec6e..8637c4d 100644
--- a/notes/todo.md
+++ b/notes/todo.md
@@ -43,6 +43,7 @@
 
 ### Assembler
 
+- [ ] Support use of `*ADDR` with =constants
 - [ ] Pad up to 256 bytes
 - [ ] Validate labels
 - [ ] Return pure machine code when printing to stdout (and not in debug mode)
diff --git a/readme.md b/readme.md
index 20c9b12..81f49b2 100644
--- a/readme.md
+++ b/readme.md
@@ -93,5 +93,8 @@ With verbose debugging output:
     
     ADD =foo        ; use a constant as an operand
 
+    LDA *ADDR       ; `*ADDR` is a magic value referencing the memory address
+                    ; that the current line will store at after assembly
+
 - Hexadecimal numbers are preceded by a `$`
 - Whitespace is ignored
\ No newline at end of file
diff --git a/test-programs/referencing-addr-during-assembly.asm b/test-programs/referencing-addr-during-assembly.asm
new file mode 100644
index 0000000..ed7031e
--- /dev/null
+++ b/test-programs/referencing-addr-during-assembly.asm
@@ -0,0 +1,11 @@
+;; Test referencing address of line being assembled
+
+LDA *ADDR
+STO $25
+FHP 0   ; hop if carry set
+ JMP @setCarry
+END
+
+@setCarry
+  FTG 0
+  JMP ($25)
\ No newline at end of file