recipes/trs80: add "recv" command

This allows us to write contents from RS-232 directly to floppy! it works!

recipes/trs80/README.md

@@ -204,4 +204,16 @@ get a usable Collapse OS prompt!
 Like with the `recv` program, nothing stops you from dumping that binary to a
 floppy.
 
-Have fun!
+## Configuration
+
+In addition to the generic basic shell, this build of Collapse OS has support
+for floppy drive `:1` as a block device (mapped to device `0`). Block device
+commands work as expected.
+
+In addition to this, there is a `flush` command to ensure that dirty buffers are
+synced to disk. Make sure you run this after a write operation or before
+swapping disks.
+
+There is also a custom `recv` command that does the same "ping pong" as in
+`recv.asm`, but once. It puts the result in `A`. This can be useful to send down
+a raw CFS: you just need a while loop that repeatedly call `recv:putb a`.

recipes/trs80/glue.asm

@@ -1,5 +1,7 @@
 ; RAMSTART is a label at the end of the file
 .equ	RAMEND		0xcfff
+; Address of the *CL driver. Same as in recv.asm
+.equ	COM_DRV_ADDR	0x0238
 
 ; Free memory in TRSDOS starts at 0x3000
 .org	0x3000
@@ -73,11 +75,61 @@ printcr:
 	pop	af
 	ret
 
+; Receive a byte from *cl and put it in A.
+; Returns A > 0xff when receiving the last byte
+recvCmd:
+	xor	a
+	ld	(VAR_TBL+1), a	; pre-set MSB
+	; put a 0xff mask in B, which will become 0x7f if we receive a 0x20
+	ld	b, 0xff
+.inner:
+	ld	a, 0x03		; @GET
+	ld	de, COM_DRV_ADDR
+	rst	0x28
+	jr	nz, .maybeerror
+	or	a
+	jr	z, .eof		; Sending a straight NULL ends the comm.
+	; @PUT that char back
+	ld	c, a
+	ld	a, 0x04		; @PUT
+	ld	de, COM_DRV_ADDR
+	rst	0x28
+	ret	nz		; error
+	ld	a, c
+	cp	0x20
+	jr	z, .escapechar
+	; not an escape char, good
+	and	b		; apply mask
+	ld	(VAR_TBL), a
+	xor	a		; ensure Z
+	ret
+.maybeerror:
+	; was it an error?
+	or	a
+	jr	z, .inner	; not an error, just loop
+	ret			; error
+.escapechar:
+	ld	b, 0x7f
+	jr	.inner
+.eof:
+	dec	a		; A = 0xff
+	ld	(VAR_TBL+1), a
+	xor	a		; ensure Z
+	ret
+
 basFindCmdExtra:
 	ld	hl, basFloppyCmds
 	call	basFindCmd
 	ret	z
 	ld	hl, basBLKCmds
+	call	basFindCmd
+	ret	z
+	ld	hl, .cmds
 	jp	basFindCmd
 
+.cmds:
+	.db	"recv", 0
+	.dw	recvCmd
+	.db	0xff		; end of table
+
 RAMSTART: