recipes/rc2014/sdcard: add instructions to write to SD card

recipes/rc2014/sdcard/README.md

@@ -90,8 +90,9 @@ World!":
 
 Then, insert your SD card in your SPI relay and boot the RC2014.
 
-Run the `sdci` command which will initialize the card. Then, run `bsel 1` to
+Run the `sdci` command which will initialize the card. The blockdev 0 is
-select the second blockdev, which is configured to be the sd card.
+already selected at initialization, but you could, to be sure, run `bsel 0` to
+select the first blockdev, which is configured to be the sd card.
 
 Set your memory pointer to somewhere you can write to with `mptr 9000` and then
 you're ready to load your contents with `load d` (load the 13 bytes that you
@@ -109,7 +110,6 @@ Then, you insert your SD card in your SPI relay and go:
 
     Collapse OS
     > sdci
-    > bsel 1
     > fson
     > fls
     helo
@@ -127,5 +127,41 @@ Now let that sink in for a minute. You've just mounted a filesystem on a SD
 card, loaded a file from it in memory and executed that file, all that on a
 kernel that weights less than 3 kilobytes!
 
+## Writing to a file in the SD card
+
+Now what we're going to do is to write back to a file on the SD card. From a
+system with the SD card initialized and the FS mounted, do:
+
+    > fopn 0 hello.txt
+    > bsel 1
+    > mptr 9000
+    9000
+    > load d
+    > peek d
+    48656C6C6F20576F726C64210A
+
+Now that we have our "Hello World!\n" loaded in memory, let's modify it and make
+it start with "XXX" and save it to the file. `sdcf` flushes the current SD card
+buffer to the card. It's automatically ran whenever we change sector during a
+read/write/seek, but was can also explicitly call it with `sdcf`.
+
+    > poke 3
+    [type "XXX"]
+    > peek d
+    5858586C6F20576F726C64210A
+    > seek 00 0000
+    0000
+    > save d
+    > sdcf
+
+The new "XXXlo World!\n" is now written to the card, at its proper place in CFS!
+You can verify this by pulling out the card (no need to unmount it from Collapse
+OS, but if you insert it again, you'll need to run `sdci` again), insert it in
+your modern system and run:
+
+    $ head -c 512 /dev/sdX | xxd
+
+You'll see your "XXXlo World!\n" somewhere, normally at offset `0x120`!
+
 [schematic]: spirelay/spirelay.pdf
 [inspiration]: https://www.ecstaticlyrics.com/electronics/SPI/fast_z80_interface.html

recipes/rc2014/sdcard/glue.asm

@@ -25,10 +25,9 @@ jp	aciaInt
 .equ	ACIA_RAMSTART	RAMSTART
 #include "acia.asm"
 .equ	BLOCKDEV_RAMSTART	ACIA_RAMEND
-.equ	BLOCKDEV_COUNT		3
+.equ	BLOCKDEV_COUNT		2
 #include "blockdev.asm"
 ; List of devices
-.dw	aciaGetC, aciaPutC, 0, 0
 .dw	sdcGetC, sdcPutC, sdcSeek, sdcTell
 .dw	blk2GetC, blk2PutC, blk2Seek, blk2Tell
 
@@ -43,9 +42,10 @@ jp	aciaInt
 #include "fs_cmds.asm"
 
 .equ	SHELL_RAMSTART		FS_RAMEND
-.equ	SHELL_EXTRA_CMD_COUNT	9
+.equ	SHELL_EXTRA_CMD_COUNT	11
 #include "shell.asm"
-.dw	sdcInitializeCmd, sdcFlushCmd, blkBselCmd, blkSeekCmd
+.dw	sdcInitializeCmd, sdcFlushCmd
+.dw	blkBselCmd, blkSeekCmd, blkLoadCmd, blkSaveCmd
 .dw	fsOnCmd, flsCmd, fnewCmd, fdelCmd, fopnCmd
 
 #include "pgm.asm"
@@ -63,14 +63,17 @@ init:
 	ld	sp, hl
 	im	1
 	call	aciaInit
-	xor	a
+	ld	hl, aciaGetC
-	ld	de, BLOCKDEV_GETC
+	ld	de, aciaPutC
-	call	blkSel
 	call	stdioInit
 	call	shellInit
 	ld	hl, pgmShellHook
 	ld	(SHELL_CMDHOOK), hl
 
+	xor	a
+	ld	de, BLOCKDEV_GETC
+	call	blkSel
+
 	ei
 	jp	shellLoop