blockdev: change GetC API

Instead of waiting, GetC always return immediately, with Z indicating if
something was fetched. The "wait" loop is implemented by the called (and
in the new `blkGetCW`).

This simplifies out-of-bounds verifications for storage blockdevs.

apps/zasm/zasm.asm

@@ -20,6 +20,7 @@ ret
 
 #include "tok.asm"
 
+; TODO: call from core
 unsetZ:
 	push	bc
 	ld	b, a

parts/acia.asm

@@ -95,17 +95,18 @@ aciaInt:
 	ei
 	reti
 
-; Read a character from the input buffer. If the buffer is empty, loop until
-; there something to fetch. Returns value in A.
+
+; *** BLOCKDEV ***
+; These function below follow the blockdev API.
+
 aciaGetC:
 	push	de
 
-.loop:
 	ld	a, (ACIA_BUFWRIDX)
 	ld	e, a
 	ld	a, (ACIA_BUFRDIDX)
 	cp	e
-	jr	z, .loop	; equal? buffer empty, wait.
+	jr	z, .nothingToRead	; equal? nothing to read.
 
 	; Alrighty, buffer not empty. let's read.
 	ld	de, ACIA_BUF
@@ -117,7 +118,12 @@ aciaGetC:
 
 	; And finally, fetch the value.
 	ld	a, (de)
+	cp	a		; ensure Z
+	jr	.end
 
+.nothingToRead:
+	call	unsetZ
+.end:
 	pop	de
 	ret
 

parts/blockdev.asm

@@ -97,13 +97,21 @@ _blkCall:
 	pop	ix
 	ret
 
-; Reads one character from selected device and returns its value in A. Always
-; returns a character and waits until read if it has to.
+; Reads one character from selected device and returns its value in A.
+; Sets Z according to whether read was successful: Set if successful, unset
+; if not.
 blkGetC:
 	ld	iyl, 0
 	jr	_blkCall
 
-; Writes character in A in current position in the selected device
+; Repeatedly call blkGetC until the call is a success.
+blkGetCW:
+	call	blkGetC
+	jr	nz, blkGetCW
+	ret
+
+; Writes character in A in current position in the selected device. Sets Z
+; according to whether the operation was successful.
 blkPutC:
 	ld	iyl, 2
 	jr	_blkCall

parts/core.asm

@@ -7,7 +7,7 @@
 ASCII_CR	.equ	0x0d
 ASCII_LF	.equ	0x0a
 
-; *** CODE ***
+; *** REGISTER FIDDLING ***
 
 ; add the value of A into DE
 addDE:
@@ -59,6 +59,17 @@ callIX:
 	jp	(ix)
 	ret
 
+; Ensures that Z is unset (more complicated than it sounds...)
+unsetZ:
+	push	bc
+	ld	b, a
+	inc	b
+	cp	b
+	pop	bc
+	ret
+
+; *** STRINGS ***
+
 ; Increase HL until the memory address it points to is null for a maximum of
 ; 0xff bytes. Returns the new HL value as well as the number of bytes iterated
 ; in A.

parts/mmap.asm

@@ -32,11 +32,13 @@ _mmapAddr:
 	ret
 
 ; if out of bounds, will continually return the last char
+; TODO: add bounds check and return Z accordingly.
 mmapGetC:
 	push	hl
 	call	_mmapAddr
 	ld	a, (hl)
 	call	_mmapForward
+	cp	a	; ensure Z
 	pop	hl
 	ret
 

parts/shell.asm

@@ -77,6 +77,7 @@ shellInit:
 shellLoop:
 	; First, let's wait until something is typed.
 	SHELL_GETC
+	jr	nz, shellLoop	; nothing typed? loop
 	; got it. Now, is it a CR or LF?
 	cp	ASCII_CR
 	jr	z, .do		; char is CR? do!