diff --git a/apps/ed/README.md b/apps/ed/README.md
index 6db9de6..19b7220 100644
--- a/apps/ed/README.md
+++ b/apps/ed/README.md
@@ -39,7 +39,8 @@ range is out of bounds.
 * `(addrs)d`: Delete lines specified in `addrs` range.
 * `(addr)a`: Appends a line after `addr`.
 * `(addr)i`: Insert a line before `addr`.
-* `q`: quit `ed`
+* `w`: write to file. For now, `q` is implied in `w`.
+* `q`: quit `ed` without writing to file.
 
 ### Current line
 
diff --git a/apps/ed/cmd.asm b/apps/ed/cmd.asm
index 473d17c..cfa761f 100644
--- a/apps/ed/cmd.asm
+++ b/apps/ed/cmd.asm
@@ -25,7 +25,9 @@
 cmdParse:
 	ld	a, (hl)
 	cp	'q'
-	jr	z, .quit
+	jr	z, .simpleCmd
+	cp	'w'
+	jr	z, .simpleCmd
 	ld	ix, CMD_ADDR1
 	call	.readAddr
 	ret	nz
@@ -67,7 +69,7 @@ cmdParse:
 	ld	(CMD_TYPE), a
 	ret			; Z already set
 
-.quit:
+.simpleCmd:
 	; Z already set
 	ld	(CMD_TYPE), a
 	ret
diff --git a/apps/ed/io.asm b/apps/ed/io.asm
index e96737b..4cf92bc 100644
--- a/apps/ed/io.asm
+++ b/apps/ed/io.asm
@@ -32,7 +32,7 @@ ioInit:
 	ld	ix, IO_FILE_HDL
 	jp	fsPutC
 .blkdev:
-	.dw	.fsGetC, unsetZ
+	.dw	.fsGetC, .fsPutC
 
 ioGetC:
 	push	ix
@@ -61,3 +61,33 @@ ioTell:
 	call	_blkTell
 	pop	ix
 	ret
+
+ioSetSize:
+	push	ix
+	ld	ix, IO_FILE_HDL
+	call	fsSetSize
+	pop	ix
+	ret
+
+; Write string (HL) in current file. Ends line with LF.
+ioPutLine:
+	push	hl
+.loop:
+	ld	a, (hl)
+	or	a
+	jr	z, .loopend		; null, we're finished
+	call	ioPutC
+	jr	nz, .error
+	inc	hl
+	jr	.loop
+.loopend:
+	; Wrote the whole line, write ending LF
+	ld	a, 0x0a
+	call	ioPutC
+	jr	z, .end		; success
+	; continue to error
+.error:
+	call	unsetZ
+.end:
+	pop	hl
+	ret
diff --git a/apps/ed/main.asm b/apps/ed/main.asm
index 1f197ff..a83c343 100644
--- a/apps/ed/main.asm
+++ b/apps/ed/main.asm
@@ -36,6 +36,7 @@
 ; fsOpen
 ; fsGetC
 ; fsPutC
+; fsSetSize
 ; intoHL
 ; printstr
 ; printcrlf
@@ -69,6 +70,8 @@ edMain:
 	ld	a, (CMD_TYPE)
 	cp	'q'
 	jr	z, .doQuit
+	cp	'w'
+	jr	z, .doWrite
 	; The rest of the commands need an address
 	call	edReadAddrs
 	jr	nz, .error
@@ -85,6 +88,26 @@ edMain:
 	xor	a
 	ret
 
+.doWrite:
+	ld	a, 3		; seek beginning
+	call	ioSeek
+	ld	de, 0		; cur line
+.writeLoop:
+	push	de \ pop hl
+	call	bufGetLine	; --> buffer in (HL)
+	jr	nz, .writeEnd
+	call	ioPutLine
+	jr	nz, .error
+	inc	de
+	jr	.writeLoop
+.writeEnd:
+	; Set new file size
+	call	ioTell
+	call	ioSetSize
+	; for now, writing implies quitting
+	; TODO: reload buffer
+	xor	a
+	ret
 .doDel:
 	; bufDelLines expects an exclusive upper bound, which is why we inc DE.
 	inc	de
@@ -114,12 +137,12 @@ edMain:
 	jr	.doPrint
 .doPrintEnd:
 	ld	(ED_CURLINE), hl
-	jr	.mainLoop
+	jp	.mainLoop
 .error:
 	ld	a, '?'
 	call	stdioPutC
 	call	printcrlf
-	jr	.mainLoop
+	jp	.mainLoop
 
 
 ; Transform an address "cmd" in IX into an absolute address in HL.
diff --git a/kernel/fs.asm b/kernel/fs.asm
index b3e28b2..4c70e8d 100644
--- a/kernel/fs.asm
+++ b/kernel/fs.asm
@@ -425,7 +425,6 @@ fsPlaceH:
 	pop	af
 	ret
 
-; Advance file handle in (IX) by one byte
 ; Sets Z according to whether HL is within bounds for file handle at (IX), that
 ; is, if it is smaller than file size.
 fsWithinBounds:
@@ -441,24 +440,20 @@ fsWithinBounds:
 .outOfBounds:
 	jp	unsetZ			; returns
 
-; Adjust, if needed, file size of handle (IX) to HL+1.
-; This adjustment only happens if this makes file size grow.
-fsAdjustBounds:
-	call	fsWithinBounds
-	ret	z
-	; Not within bounds? let's increase them
-	push	hl
+; Set size of file handle (IX) to value in HL.
+; This writes directly in handle's metadata.
+fsSetSize:
+	push	hl		; --> lvl 1
 	ld	hl, 0
 	call	fsPlaceH	; fs blkdev is now at beginning of content
 	; we need the blkdev to be on filesize's offset
 	ld	hl, FS_METASIZE-FS_META_FSIZE_OFFSET
 	ld	a, BLOCKDEV_SEEK_BACKWARD
 	call	fsblkSeek
-	pop	hl
+	pop	hl		; <-- lvl 1
 	; blkdev is at the right spot, HL is back to its original value, let's
-	; write it.
-	push	hl
-	inc	hl	; We write HL+1, remember
+	; write it both in the metadata block and in its file handle's cache.
+	push	hl		; --> lvl 1
 	; now let's write our new filesize both in blkdev and in file handle's
 	; cache.
 	ld	a, l
@@ -467,7 +462,7 @@ fsAdjustBounds:
 	ld	a, h
 	ld	(ix+5), a
 	call	fsblkPutC
-	pop	hl
+	pop	hl		; <-- lvl 1
 	xor	a	; ensure Z
 	ret
 
@@ -488,7 +483,7 @@ fsGetC:
 	pop	hl
 	ret
 
-; Write byte A in handle (IX) and advance the handle's position.
+; Write byte A in handle (IX) at position HL.
 ; Z is set on success, unset if handle is at the end of the file.
 ; TODO: detect end of block alloc
 fsPutC:
@@ -496,7 +491,11 @@ fsPutC:
 	call	fsPlaceH
 	call	fsblkPutC
 	pop	hl
-	jp	fsAdjustBounds	; returns
+	; if HL is out of bounds, increase bounds
+	call	fsWithinBounds
+	ret	z
+	inc	hl		; our filesize is now HL+1
+	jp	fsSetSize
 
 ; Mount the fs subsystem upon the currently selected blockdev at current offset.
 ; Verify is block is valid and error out if its not, mounting nothing.
diff --git a/tools/emul/shell/shell_.asm b/tools/emul/shell/shell_.asm
index a522124..acc61c2 100644
--- a/tools/emul/shell/shell_.asm
+++ b/tools/emul/shell/shell_.asm
@@ -28,6 +28,7 @@
 	jp	fsOpen
 	jp	fsGetC
 	jp	fsPutC
+	jp	fsSetSize
 	jp	cpHLDE
 	jp	parseArgs
 	jp	printstr
diff --git a/tools/emul/shell/user.h b/tools/emul/shell/user.h
index c2d1379..43855ff 100644
--- a/tools/emul/shell/user.h
+++ b/tools/emul/shell/user.h
@@ -21,13 +21,14 @@
 .equ	fsOpen			0x2d
 .equ	fsGetC			0x30
 .equ	fsPutC			0x33
-.equ	cpHLDE			0x36
-.equ	parseArgs		0x39
-.equ	printstr		0x3c
-.equ	_blkGetC		0x3f
-.equ	_blkPutC		0x42
-.equ	_blkSeek		0x45
-.equ	_blkTell		0x48
-.equ	printcrlf		0x4b
-.equ	stdioPutC		0x4e
-.equ	stdioReadLine	0x51
+.equ	fsSetSize		0x36
+.equ	cpHLDE			0x39
+.equ	parseArgs		0x3c
+.equ	printstr		0x3f
+.equ	_blkGetC		0x42
+.equ	_blkPutC		0x45
+.equ	_blkSeek		0x48
+.equ	_blkTell		0x4b
+.equ	printcrlf		0x4e
+.equ	stdioPutC		0x51
+.equ	stdioReadLine	0x54