Optimize EXIT, C!, C@ and 0= (#436)

control.asm

@@ -113,13 +113,13 @@ EXIT
     lda HERE_LSB
     sec
     sbc #3
-    sta .instr_ptr
+    tay
     lda HERE_MSB
     sbc #0
     sta .instr_ptr + 1
     lda #OP_JMP
 .instr_ptr = * + 1
-    sta PLACEHOLDER_ADDRESS ; replaced with instruction pointer
+    sta PLACEHOLDER_ADDRESS,y ; replaced with instruction pointer
     rts
 +
     lda #OP_RTS

core.asm

@@ -123,10 +123,10 @@ EQUAL
     +BACKLINK "0=", 2
 ZEQU
     ldy #0
-    lda MSB, x
-    bne +
     lda LSB, x
     bne +
+    lda MSB, x
+    bne +
     dey
 +   sty MSB, x
     sty LSB, x
@@ -179,23 +179,21 @@ FETCH
 
     +BACKLINK "c!", 2
 STOREBYTE
-    lda LSB,x
-    sta + + 1
+    ldy LSB,x
     lda MSB,x
     sta + + 2
     lda	LSB+1,x
-+   sta PLACEHOLDER_ADDRESS ; replaced with addr
++   sta PLACEHOLDER_ADDRESS,y ; replaced with addr
     inx
     inx
     rts
 
     +BACKLINK "c@", 2
 FETCHBYTE
-    lda LSB,x
-    sta + + 1
+    ldy LSB,x
     lda MSB,x
     sta + + 2
-+   lda PLACEHOLDER_ADDRESS ; replaced with addr
++   lda PLACEHOLDER_ADDRESS,y ; replaced with addr
     sta LSB,x
     lda #0
     sta MSB,x

durexforth.asm

@@ -68,7 +68,8 @@ K_CLRSCR = $93
 K_SPACE = ' '
 
 ; PLACEHOLDER_ADDRESS instances are overwritten using self-modifying code.
-PLACEHOLDER_ADDRESS = $1234
+; It must end in 00 for situations where the Y register is used as the LSB of the address.
+PLACEHOLDER_ADDRESS = $1200
 
 !ct pet