z80mbc2: add BLK support

ref #82

recipes/z80mbc2/Makefile

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+TARGET = os.bin
+BASEDIR = ../..
+EDIR = $(BASEDIR)/emul
+STAGE = $(EDIR)/stage
+
+.PHONY: all
+all: $(TARGET) 
+$(TARGET): xcomp.fs $(STAGE)
+	cat xcomp.fs | $(STAGE) > $@
+
+$(STAGE):
+	$(MAKE) -C $(EDIR) stage
+

recipes/z80mbc2/README.md

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+# Z80-MBC2
+
+The [Z80-MBC2][link] combines a Z80 and an ATMEGA32A to provide a CP/m capable
+computing environment. It features a SD card bootloader which makes running
+Collapse OS on it rather simple.
+
+## Recipe
+
+In this recipe, we're going to run Collapse OS on the Z80-MBC2, interfacing
+through its serial port. We're going to use the MBC's API to implement BLK on
+the SD card.
+
+### Gathering parts
+
+* A Z80-MBC2 computer with its SD card module and a properly flashed "IOS" on
+  the ATMega32A.
+* A FTDI-to-TTL cable to connect to the serial port.
+
+### Building the binary
+
+Running `make` will yield `os.bin` which is what we want.
+
+### Running on the Z80-MBC2
+
+Mount the SD card on your modern computer and copy `os.bin` as `autoboot.bin`,
+overwriting the binary that was previously there.
+
+We also have to copy the blkfs over. This is done by using IOS' drive system.
+Each `DSxNyy.DSK` file on the card is a drive, each drive has 512 track of 32
+sectors of 512 bytes, so one drive is plenty for our needs. Collapse OS
+hardcodes drive 0.
+
+Each drive is part of a set. IOS theoretically supports up to 10 sets, but the
+binary shipped by default only accepts 4. You have to overwrite an existing set.
+I used set 3. So, copy `blkfs` to file `DS3N00.DSK`. If you want, you can change
+the name of the set by changing the contents of `DS3NAM.DAT`.
+
+Put back the SD card in the Z80-MBC2 and power it up by connecting the FTDI
+adapter to it (red: VCC, black: GND, green: TX, white: RX).
+
+The FTDI adapter will show up as something like `ttyUSB0` (or `ttyU0` on
+OpenBSD). Connect to it with `screen` or `cu` or whatever you like. Baud rate of
+the Z80-MBC2 appears to be hardcoded to 115200.
+
+Then, enable IOS program selection by holding RESET and USER at the same time,
+wait 2 seconds, releasing RESET, wait 2 seconds, releasing USER. You should then
+be given a 1-8 choice.
+
+You begin by selecting the proper disk set, which is through choice 8, then you
+select the Autoboot binary through choice 4.
+
+You are now in Collapse OS.
+
+[link]: https://hackaday.io/project/159973-z80-mbc2-a-4-ics-homebrew-z80-computer

recipes/z80mbc2/xcomp.fs

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+0xff00 CONSTANT RS_ADDR        0xfffa CONSTANT PS_ADDR
+RS_ADDR 0x80 - CONSTANT RAMSTART
+212 LOAD  ( z80 assembler )
+262 LOAD  ( xcomp )            270 LOAD  ( xcomp overrides )
+282 LOAD  ( boot.z80 )         353 LOAD  ( xcomp core low )
+CODE (emit)
+    A 1 LDrn, 1 OUTnA, HL POPqq, A L LDrr, 0 OUTnA,
+;CODE
+CODE (key)
+    BEGIN, 1 INAn, A INCr, JRZ, AGAIN,
+    A DECr, PUSHA,
+;CODE
+: _sel ( sec )
+( 32 sectors per track, 512 tracks per disk )
+    32 /MOD ( addr sec trk )
+    0x0a ( seltrk ) 1 PC! 0 PC! 0 0 PC! ( addr sec )
+    0x0b ( selsec ) 1 PC! 0 PC! ( addr ) ;
+: _ ( addr )
+    ( get 512 bytes )
+    0x86 ( readsec ) 1 PC!
+    512 0 DO 0 PC@ SWAP C!+ LOOP DROP
+;
+: FD@ 2 * DUP _sel BLK( _ 1+ _sel BLK( 512 + _ ;
+: _ ( addr )
+    ( write 512 bytes )
+    0x0c ( writesec ) 1 PC!
+    512 0 DO C@+ 0 PC! LOOP DROP
+;
+: FD! 2 * DUP _sel BLK( _ 1+ _sel BLK( 512 + _ ;
+: FD$ ( select disk 0 )
+    0x09 ( seldisk ) 1 PC! 0 0 PC! ( sel disk 0 )
+;
+380 LOAD  ( xcomp core high )
+(entry) _
+( Update LATEST )
+PC ORG @ 8 + !
+," CURRENT @ HERE ! BLK$ FD$ ' FD@ BLK@* ! ' FD! BLK!* ! " EOT,
+ORG @ 256 /MOD 2 PC! 2 PC!
+H@ 256 /MOD 2 PC! 2 PC!