diff --git a/apps/zasm/directive.asm b/apps/zasm/directive.asm
index 4c9ab2b..229839c 100644
--- a/apps/zasm/directive.asm
+++ b/apps/zasm/directive.asm
@@ -50,12 +50,16 @@ handleDB:
 	jr	nz, .overflow	; not zero? overflow
 	ld	a, l
 	call	ioPutC
+	jr	nz, .ioError
 .stopStrLit:
 	call	readComma
 	jr	z, .loop
 	cp	a		; ensure Z
 	pop	hl
 	ret
+.ioError:
+	ld	a, SHELL_ERR_IO_ERROR
+	jr	.error
 .badfmt:
 	ld	a, ERR_BAD_FMT
 	jr	.error
diff --git a/apps/zasm/instr.asm b/apps/zasm/instr.asm
index 46346ea..c20e1d0 100644
--- a/apps/zasm/instr.asm
+++ b/apps/zasm/instr.asm
@@ -823,10 +823,14 @@ parseInstruction:
 .loopWrite:
 	ld	a, (hl)
 	call	ioPutC
+	jr	nz, .ioError
 	inc	hl
 	djnz	.loopWrite
 	cp	a	; ensure Z
 	jr	.end
+.ioError:
+	ld	a, SHELL_ERR_IO_ERROR
+	jr	.error
 .overflow:
 	ld	a, ERR_OVFL
 	jr	.error
diff --git a/apps/zasm/io.asm b/apps/zasm/io.asm
index dff7c7a..c02d096 100644
--- a/apps/zasm/io.asm
+++ b/apps/zasm/io.asm
@@ -152,6 +152,7 @@ ioPutC:
 	jp	_blkPutC
 .skip:
 	pop	af
+	cp	a		; ensure Z
 	ret
 
 ioSavePos:
diff --git a/kernel/sdc.asm b/kernel/sdc.asm
index 48cfb90..ddced67 100644
--- a/kernel/sdc.asm
+++ b/kernel/sdc.asm
@@ -529,11 +529,13 @@ sdcSync:
 .buf1Ok:
 	ld	de, SDC_BUFSEC1
 	ld	(SDC_BUFPTR), de
+	; Z already set
 	jr	.end
 
 .buf2Ok:
 	ld	de, SDC_BUFSEC2
 	ld	(SDC_BUFPTR), de
+	; Z already set
 	; to .end
 .end:
 	pop	de
diff --git a/tools/emul/zasm/glue.asm b/tools/emul/zasm/glue.asm
index d86f7ba..97be16e 100644
--- a/tools/emul/zasm/glue.asm
+++ b/tools/emul/zasm/glue.asm
@@ -82,6 +82,7 @@ emulGetC:
 
 emulPutC:
 	out	(STDIO_PORT), a
+	cp	a		; ensure Z
 	ret
 
 fsdevGetC:
@@ -111,6 +112,7 @@ fsdevPutC:
 	jr	nz, .error
 	pop	af
 	out	(FS_DATA_PORT), a
+	cp	a		; ensure Z
 	ret
 .error:
 	pop	af