zasm: add support for subtractions in expressions

apps/zasm/expr.asm

@@ -2,34 +2,69 @@
 ; We expect (HL) to be disposable: we mutate it to avoid having to make a copy.
 ; Sets Z on success, unset on error.
 parseExpr:
+	push	bc
+	push	de
 	push	hl
 	ld	a, '+'
 	call	JUMP_FINDCHAR
+	jr	z, .hasExpr
+	pop	hl
+	push	hl
+	ld	a, '-'
+	call	JUMP_FINDCHAR
 	jr	nz, .noExpr
-	; Alright, we have a + and we're pointing at it. Let's advance HL and
+	ld	c, '-'
+	jr	.hasExpr
+.hasPlus:
+	ld	c, '+'
+	jr	.hasExpr
+.hasExpr:
+	; Alright, we have a +/ and we're pointing at it. Let's advance HL and
 	; recurse. But first, let's change this + into a null char. It will be
 	; handy later.
 	xor	a
 	ld	(hl), a		; + changed to \0
 
 	inc	hl
-	call	parseExpr
-	; Whether parseExpr was successful or not, we pop hl right now
-	pop	hl
-	ret	nz		; return immediately if error
-	; Now we have parsed everything to the right and we have its result in
-	; IX. the pop hl brought us back to the beginning of the string. Our
-	; + was changed to a 0. Let's save IX somewhere and parse this.
-	push	de
-	ld	d, ixh
-	ld	e, ixl
-	call	parseNumberOrSymbol
-	jr	nz, .end		; error
-	; Good! let's do the math!
-	add	ix, de
-.end:
+	pop	de		; we pop out the HL we pushed earlier into DE
+				; That's our original beginning of string.
+	call	_applyExprToHL
 	pop	de
+	pop	bc
 	ret
+
 .noExpr:
 	pop	hl
+	pop	de
+	pop	bc
 	jp	parseNumberOrSymbol
+
+; Parse number or symbol in (DE) and expression in (HL) and apply operator
+; specified in C to them.
+_applyExprToHL:
+	call	parseExpr
+	ret	nz		; return immediately if error
+	; Now we have parsed everything to the right and we have its result in
+	; IX. What we need to do now is parseNumberOrSymbol on (DE) and apply
+	; operator. Let's save IX somewhere and parse this.
+	ex	hl, de
+	push	ix
+	pop	de
+	call	parseNumberOrSymbol
+	ret	nz		; error
+	; Good! let's do the math! IX has our left part, DE has our right one.
+	ld	a, c		; restore operator
+	cp	'-'
+	jr	z, .sub
+	; addition
+	add	ix, de
+	jr	.end
+.sub:
+	push	ix
+	pop	hl
+	sbc	hl, de
+	push	hl
+	pop	ix
+.end:
+	cp	a		; ensure Z
+	ret

apps/zasm/tests/test5.asm

@@ -1,2 +1,3 @@
 ; test expressions
 ld	a, 'A'+3
+ld	a, 'A'-0x20