shell: add "seek" and "peek" dummy commands

This change introduces a command name matching algo.

parts/shell.asm

@@ -4,34 +4,59 @@
 ; for now, this unit is tightly coupled to acia.asm, but it will eventually be
 ; more general than that.
 
-; Incomplete. For now, this outputs a welcome prompt and then waits for input.
-; Whenever input is CR or LF, we echo back what we've received and empty the
-; input buffer. This also happen when the buffer is full.
+; Status: incomplete. As it is now, it spits a welcome prompt, wait for input
+; and compare the first 4 chars of the input with a command table and call the
+; appropriate routine if it's found, an error if it's not.
+;
+; Commands, for now, are dummy.
+;
+; See constants below for error codes.
 
 ; *** CONSTS ***
 CR	.equ	0x0d
 LF	.equ	0x0a
 
+; number of entries in shellCmdTbl
+SHELL_CMD_COUNT	.equ	2
+
+; The command that was type isn't known to the shell
+SHELL_ERR_UNKNOWN_CMD	.equ	0x01
+
+; Arguments for the command weren't properly formatted
+SHELL_ERR_BAD_ARGS	.equ	0x02
+
+; *** CODE ***
 shellInit:
 	; print prompt
-	ld	hl, d_welcome
+	ld	hl, .prompt
 	call	printstr
 	call	printcrlf
 	ret
 
+.prompt:
+	.db	"Collapse OS", 0
+
 shellLoop:
 	call	chkbuf
 	jr	shellLoop
 
 ; print null-terminated string pointed to by HL
 printstr:
+	push	af
+	push	hl
+
+.loop:
 	ld	a, (hl)		; load character to send
 	or	a		; is it zero?
-	ret	z		; if yes, we're finished
+	jr	z, .end		; if yes, we're finished
 	call	aciaPutC
 	inc	hl
-	jr	printstr
-	; no ret because our only way out is ret z above
+	jr	.loop
+
+.end:
+	pop	hl
+	pop	af
+	ret
 
 printcrlf:
 	ld	a, CR
@@ -73,9 +98,124 @@ chkbuf:
 
 	; alright, let's go!
 	ld	hl, ACIA_BUF
+	call	shellParse
+	ret
+
+; Compares strings pointed to by HL and DE up to A count of characters. If
+; equal, Z is set. If not equal, Z is reset.
+strncmp:
+	push	bc
+	push	hl
+	push	de
+
+	ld	b, a
+.loop:
+	ld	a, (de)
+	cp	(hl)
+	jr	nz, .end	; not equal? break early
+	inc	hl
+	inc	de
+	djnz	.loop
+
+.end:
+	pop	de
+	pop	hl
+	pop	bc
+	; Because we don't call anything else than CP that modify the Z flag,
+	; our Z value will be that of the last cp (reset if we broke the loop
+	; early, set otherwise)
+	ret
+
+; add the value of A into DE
+addDE:
+	add	a, e
+	jr	nc, .end	; no carry? skip inc
+	inc	d
+.end:
+	ld	e, a
+	ret
+
+; jump to the location pointed to by HL. This allows us to call HL instead of
+; just jumping it.
+jumpHL:
+	jp	hl
+	ret
+
+; Parse command (null terminated) at HL and calls it
+shellParse:
+	push	af
+	push	bc
+	push	de
+
+	ld	de, shellCmdTbl
+	ld	a, SHELL_CMD_COUNT
+	ld	b, a
+
+.loop:
+	ld	a, 4		; 4 chars to compare
+	call	strncmp
+	jr	z, .found
+	ld	a, 6
+	call	addDE
+	djnz	.loop
+
+	; exhausted loop? not found
+	ld	a, SHELL_ERR_UNKNOWN_CMD
+	call	shellPrintErr
+	jr	.end
+
+.found:
+	ld	a, 4
+	call	addDE
+	ex	hl, de
+	call	jumpHL
+	ex	hl, de
+
+.end:
+	pop	de
+	pop	bc
+	pop	af
+	ret
+
+; Print the error code set in A (doesn't work for codes > 9 yet...)
+shellPrintErr:
+	push	af
+	push	hl
+
+	ld	hl, .str
 	call	printstr
+
+	; ascii for '0' is 0x30
+	add	a, 0x30
+	call	aciaPutC
 	call	printcrlf
+
+	pop	hl
+	pop	af
+	ret
+
+.str:
+	.db	"ERR ", 0
+
+; *** COMMANDS ***
+shellSeek:
+	ld	hl, .str
+	call	printstr
 	ret
+.str:
+	.db	"seek called", CR, LF, 0
+
+shellPeek:
+	ld	hl, .str
+	call	printstr
+	ret
+.str:
+	.db	"peek called", CR, LF, 0
+
+; Format: 4 bytes name followed by 2 bytes jump. fill names with zeroes
+shellCmdTbl:
+	.db	"seek"
+	jr	shellSeek
+	.db	"peek"
+	jr	shellPeek
 
-; *** DATA ***
-d_welcome:	.byte	"Welcome to Collapse OS", 0