zasm: have .fill generate an error on overflow

Can possibly avoid a lot of debugging pain.

apps/zasm/README.md

@@ -130,6 +130,11 @@ allowed. An included file cannot have an `.inc` directive.
            offset. For example, if we want to place an instruction exactly at
            byte 0x38, we would precede it with `.fill 0x38-$`.
 
+The maximum value possible for `.fill` is `0xd000`. We do this to
+avoid "overshoot" errors, that is, error where `$` is greater than
+the offset you're trying to reach in an expression like `.fill X-$`
+(such an expression overflows to `0xffff`).
+
 **.org**: Sets the Program Counter to the value of the argument, an expression.
           For example, a label being defined right after a `.org 0x400`, would
           have a value of `0x400`. Does not do any filling. You have to do that

apps/zasm/directive.asm

@@ -201,8 +201,11 @@ handleFIL:
 	jr	nz, .badfmt
 	call	parseExpr
 	jr	nz, .badarg
-	push	bc
+	push	bc	; --> lvl 1
 	push	ix \ pop bc
+	ld	a, b
+	cp	0xd0
+	jr	nc, .overflow
 .loop:
 	ld	a, b
 	or	c
@@ -214,19 +217,21 @@ handleFIL:
 	jr	.loop
 .loopend:
 	cp	a	; ensure Z
-	pop	bc
+	pop	bc	; <-- lvl 1
 	ret
 .ioError:
 	ld	a, SHELL_ERR_IO_ERROR
-	jr	.error
+	jp	unsetZ
 .badfmt:
 	ld	a, ERR_BAD_FMT
-	jr	.error
+	jp	unsetZ
 .badarg:
 	ld	a, ERR_BAD_ARG
-.error:
+	jp	unsetZ
-	call	unsetZ
+.overflow:
-	ret
+	pop	bc	; <-- lvl 1
+	ld	a, ERR_OVFL
+	jp	unsetZ
 
 handleOUT:
 	push	hl

tools/tests/zasm/errtests.sh

@@ -54,6 +54,7 @@ chkerr ".inc" 19
 chkerr ".inc foo" 19
 chkerr "ld a, 0x100" 20
 chkerr ".db 0x100" 20
+chkerr $'nop \ nop \ nop\n.fill 2-$' 20
 chkerr ".inc \"doesnotexist\"" 21
 chkerr "foo:\\foo:" 22
 chkoom