Added BYTE REGISTER to docs and to editor syntaxes

commands.md

@@ -113,12 +113,14 @@ WEND
 
 ## BYTE
 
-Declares an 8-bit variable or constant.
+Declares an 8-bit variable, register-hinted temporary, or constant.
 
 **Syntax:**
 ```
 BYTE <varname>
 BYTE <varname> = <value>
+BYTE REGISTER <varname>
+BYTE REGISTER <varname> = <value>
 BYTE <varname> @ <address>
 BYTE CONST <varname> = <value>
 ```
@@ -127,10 +129,17 @@ BYTE CONST <varname> = <value>
 ```
 BYTE counter
 BYTE speed = 5
+BYTE REGISTER temp
+BYTE REGISTER scratch = 0
 BYTE screen @ $D020
 BYTE CONST MAX_SPEED = 10
 ```
 
+`REGISTER` is a storage hint for the optimizer. The variable may be kept in a
+CPU register (A/X/Y) and its memory allocation can be eliminated entirely.
+Only valid inside `FUNC`/`FEND` blocks; incompatible with `@` and `CONST`.
+See the REGISTER section in `language.md` for details.
+
 ---
 
 ## CALL

editor_syntaxes/kate/c65cm.xml

@@ -43,6 +43,7 @@
       <item>POINTER</item>
       <item>POKE</item>
       <item>POKEW</item>
+      <item>REGISTER</item>
       <item>SCRIPT</item>
       <item>STEP</item>
       <item>SUBEND</item>

editor_syntaxes/sublime/c65cm.sublime-syntax

@@ -38,7 +38,7 @@ contexts:
       scope: meta.preprocessor.c65cm
 
   keywords:
-    - match: '\b(ADD|AND|AS|ASM|BREAK|BYTE|CALL|CASE|CONST|DEC|DECREMENT|DEFAULT|DO|ELSE|ENDASM|ENDIF|ENDSCRIPT|ENDSWITCH|EXIT|FEND|FOR|FUNC|GOSUB|GOTO|IF|INC|INCREMENT|LABEL|LET|LIBRARY|MACRO|NEXT|OR|ORIGIN|PASSING|PEEK|PEEKW|POINTER|POKE|POKEW|SCRIPT|STEP|SUBEND|SUBTRACT|SWITCH|THEN|TO|WHILE|WITH|WEND|WORD|XOR)\b'
+    - match: '\b(ADD|AND|AS|ASM|BREAK|BYTE|CALL|CASE|CONST|DEC|DECREMENT|DEFAULT|DO|ELSE|ENDASM|ENDIF|ENDSCRIPT|ENDSWITCH|EXIT|FEND|FOR|FUNC|GOSUB|GOTO|IF|INC|INCREMENT|LABEL|LET|LIBRARY|MACRO|NEXT|OR|ORIGIN|PASSING|PEEK|PEEKW|POINTER|POKE|POKEW|REGISTER|SCRIPT|STEP|SUBEND|SUBTRACT|SWITCH|THEN|TO|WHILE|WITH|WEND|WORD|XOR)\b'
       scope: keyword.control.c65cm
     - match: '\b(in|out|io)\b(?=\s*:)'
       scope: storage.modifier.parameter.c65cm

internal/optimizer/optimizer_test.go

@@ -732,3 +732,36 @@ func TestPassRegDead_CallSiteKeepsStore(t *testing.T) {
 		t.Errorf("expected sta call_val to be kept (before jsr), got:\n%s", joined)
 	}
 }
+
+func TestPassRegDead_InitValueFlowsThrough(t *testing.T) {
+	// BYTE REGISTER x = 42; POKE $d020, x → value flows through A, never touches RAM
+	input := []string{
+		"\tlda #$2a",
+		"\tsta test_x",
+		"\tlda test_x",
+		"\tsta 53280",
+		"\trts",
+	}
+
+	cfg := &Config{
+		EnableRegisterVars: true,
+		EnableStoreLoad:    true,
+		RegisterVars: map[string]bool{
+			"test_x": true,
+		},
+	}
+
+	output, dissolved := Optimize(input, cfg)
+
+	if !dissolved["test_x"] {
+		t.Error("test_x should be dissolved — init value flows through A to POKE")
+	}
+
+	joined := strings.Join(output, "\n")
+	if strings.Contains(joined, "test_x") {
+		t.Errorf("expected no reference to test_x in output, got:\n%s", joined)
+	}
+	if !strings.Contains(joined, "lda #$2a") {
+		t.Error("init value lda #$2a should survive")
+	}
+}

language.md

@@ -88,12 +88,40 @@ initialize()
 BYTE variables store 8-bit values (0-255).
 
 ```c65
-BYTE count                    // Uninitialized
+BYTE count                    // Uninitialized (0)
 BYTE speed = 5               // Initialized to 5
+BYTE REGISTER temp           // Register-hinted (default 0)
+BYTE REGISTER scratch = 0    // Register-hinted with init value
 BYTE screen @ $D020          // Memory-mapped to specific address
-BYTE CONST MAX_SPEED = 10    // Constant (recommended over #DEFINE)
+BYTE CONST MAX_SPEED = 10    // Constant
 ```
 
+### REGISTER Variables
+
+The `REGISTER` hint tells the optimizer that a BYTE variable's value only matters as
+it flows through the computation — never its stored location. The compiler can keep
+the value in a CPU register (A, X, or Y) and eliminate dead stores and memory
+allocations entirely.
+
+```c65
+FUNC fast_copy
+  BYTE REGISTER b
+  b = PEEK $d011
+  b = b | 32
+  POKE $d011, b             // b never touches RAM — flows through A
+FEND
+```
+
+**Rules:**
+
+- `BYTE REGISTER` is only valid inside `FUNC`/`FEND` blocks (function-local only)
+- Cannot be combined with `@` (memory-mapped) or `CONST`
+- Cannot be referenced from `ASM`, `SCRIPT`, or `MACRO` blocks via `|varname|`
+- `WORD REGISTER` is not supported (the 6502 has no 16-bit ALU register)
+- Without `--opt`, `REGISTER` behaves identically to a normal `BYTE`
+- With `--opt`, the optimizer may dissolve the variable, eliminating its `!8`
+  allocation and all store/load operations
+
 ### WORD Variables
 
 WORD variables store 16-bit values (0-65535).
@@ -109,7 +137,7 @@ WORD CONST SCREEN_RAM = $0400   // Constant
 
 ### Memory-Mapped Variables
 
-Variables can be placed at specific addresses using `@`:
+Variables can be placed at specific addresses using `@`. Not compatible with `REGISTER`.
 
 ```c65
 BYTE borderColor @ $D020        // VIC-II border color