Separate shell and acia input buffers

They serve a different purpose. The goal of the ACIA buffer is to ensure
that we don't miss an input. The goal of the shell buffer is to wait
until the user presses return.

The ACIA buffer has been moved to shell and replaced with a circular
buffer, a more appropriate data structure for this kind of purpose.

Also, introduce `aciaGetC`.

parts/acia.asm

@@ -14,23 +14,29 @@
 ;                RAM.
 
 ; *** CONSTS ***
-; size of the input buffer. If our input goes over this size, we echo
-; immediately.
+; size of the input buffer. If our input goes over this size, we start losing
+; data.
 ACIA_BUFSIZE	.equ	0x20
 
 ; *** VARIABLES ***
-; Our input buffer starts there
+; Our input buffer starts there. This is a circular buffer.
 ACIA_BUF	.equ	ACIA_RAMSTART
 
-; index, in the buffer, where our next character will go. 0 when the buffer is
-; empty, BUFSIZE-1 when it's almost full.
-ACIA_BUFIDX	.equ	ACIA_BUF+ACIA_BUFSIZE
-ACIA_RAMEND	.equ	ACIA_BUFIDX+1
+; The "read" index of the circular buffer. It points to where the next char
+; should be read. If rd == wr, the buffer is empty. Not touched by the
+; interrupt.
+ACIA_BUFRDIDX	.equ	ACIA_BUF+ACIA_BUFSIZE
+; The "write" index of the circular buffer. Points to where the next char
+; should be written. Should only be touched by the interrupt. if wr == rd-1,
+; the interrupt will *not* write in the buffer until some space has been freed.
+ACIA_BUFWRIDX	.equ	ACIA_BUFRDIDX+1
+ACIA_RAMEND	.equ	ACIA_BUFWRIDX+1
 
 aciaInit:
 	; initialize variables
 	xor	a
-	ld	(ACIA_BUFIDX), a	; starts at 0
+	ld	(ACIA_BUFRDIDX), a	; starts at 0
+	ld	(ACIA_BUFWRIDX), a
 
 	; setup ACIA
 	; CR7 (1) - Receive Interrupt enabled
@@ -41,6 +47,16 @@ aciaInit:
 	out	(ACIA_CTL), a
 	ret
 
+; Increase the circular buffer index in A, properly considering overflow.
+; returns value in A.
+aciaIncIndex:
+	inc	a
+	cp	ACIA_BUFSIZE
+	ret	nz	; not equal? nothing to do
+	; equal? reset
+	xor	a
+	ret
+
 ; read char in the ACIA and put it in the read buffer
 aciaInt:
 	push	af
@@ -51,18 +67,27 @@ aciaInt:
 	bit	0, a		; is our ACIA rcv buffer full?
 	jr	z, .end		; no? a interrupt was triggered for nothing.
 
-	call	aciaBufPtr	; HL set, A set
-	; is our input buffer full? If yes, we don't read anything. Something
-	; is wrong: we don't process data fast enough.
-	cp	ACIA_BUFSIZE
-	jr	z, .end		; if BUFIDX == BUFSIZE, our buffer is full.
+	; Load both read and write indexes so we can compare them. To do so, we
+	; perform a "fake" read increase and see if it brings it to the same
+	; value as the write index.
+	ld	a, (ACIA_BUFRDIDX)
+	call	aciaIncIndex
+	ld	l, a
+	ld	a, (ACIA_BUFWRIDX)
+	cp	l
+	jr	z, .end		; Equal? buffer is full
 
+	; Alrighty, buffer not full. let's write.
+	ld	de, ACIA_BUF
+	; A already contains our write index, add it to DE
+	call	addDE
 	; increase our buf ptr while we still have it in A
-	inc	a
-	ld	(ACIA_BUFIDX), a
+	call	aciaIncIndex
+	ld	(ACIA_BUFWRIDX), a
 
+	; And finially, fetch the value and write it.
 	in	a, (ACIA_IO)
-	ld	(hl), a
+	ld	(de), a
 
 .end:
 	pop	hl
@@ -70,18 +95,30 @@ aciaInt:
 	ei
 	reti
 
-; Set current buffer pointer in HL. The buffer pointer is where our *next* char
-; will be written. A is set to the value of (BUFIDX)
-aciaBufPtr:
-	push	bc
+; Read a character from the input buffer. If the buffer is empty, loop until
+; there something to fetch. Returns value in A.
+aciaGetC:
+	push	de
 
-	ld	a, (ACIA_BUFIDX)
-	ld	hl, ACIA_BUF
-	xor	b
-	ld	c, a
-	add	hl, bc		; hl now points to INPTBUF + BUFIDX
+.loop:
+	ld	a, (ACIA_BUFWRIDX)
+	ld	e, a
+	ld	a, (ACIA_BUFRDIDX)
+	cp	e
+	jr	z, .loop	; equal? buffer empty, wait.
 
-	pop	bc
+	; Alrighty, buffer not empty. let's read.
+	ld	de, ACIA_BUF
+	; A already contains our read index, add it to DE
+	call	addDE
+	; increase our buf ptr while we still have it in A
+	call	aciaIncIndex
+	ld	(ACIA_BUFRDIDX), a
+
+	; And finially, fetch the value.
+	ld	a, (de)
+
+	pop	de
 	ret
 
 ; spits character in A in port SER_OUT

parts/core.asm

@@ -18,6 +18,26 @@ addDE:
 	ld	e, a
 	ret
 
+; Increase HL until the memory address it points to is null for a maximum of
+; 0xff bytes. Returns the new HL value as well as the number of bytes iterated
+; in A.
+findnull:
+	push	bc
+	ld	a, 0xff
+	ld	b, a
+
+.loop:	ld	a, (hl)
+	cp	0
+	jr	z, .end
+	inc	hl
+	djnz	.loop
+.end:
+	; We ran 0xff-B loops. That's the result that goes in A.
+	ld	a, 0xff
+	sub	a, b
+	pop	bc
+	ret
+
 ; Format the lower nibble of A into a hex char and stores the result in A.
 fmtHex:
 	and	a, 0xf
@@ -191,3 +211,4 @@ upcase:
 	; 'a' - 'A' == 0x20
 	sub	0x20
 	ret
+

parts/shell.asm

@@ -26,18 +26,29 @@ SHELL_ERR_UNKNOWN_CMD	.equ	0x01
 ; Arguments for the command weren't properly formatted
 SHELL_ERR_BAD_ARGS	.equ	0x02
 
+; Size of the shell command buffer. If a typed command reaches this size, the
+; command is flushed immediately (same as pressing return).
+SHELL_BUFSIZE		.equ	0x20
+
 ; *** VARIABLES ***
 ; Memory address that the shell is currently "pointing at" for peek and deek
 ; operations. Set with seek.
 SHELL_MEM_PTR	.equ	SHELL_RAMSTART
 ; Used to store formatted hex values just before printing it.
 SHELL_HEX_FMT	.equ	SHELL_MEM_PTR+2
-SHELL_RAMEND	.equ	SHELL_HEX_FMT+2
+
+; Command buffer. We read types chars into this buffer until return is pressed
+; This buffer is null-terminated and we don't keep an index around: we look
+; for the null-termination every time we write to it. Simpler that way.
+SHELL_BUF	.equ	SHELL_HEX_FMT+2
+
+SHELL_RAMEND	.equ	SHELL_BUF+SHELL_BUFSIZE
 
 ; *** CODE ***
 shellInit:
 	xor	a
 	ld	(SHELL_MEM_PTR), a
+	ld	(SHELL_BUF), a
 
 	; print prompt
 	ld	hl, .prompt
@@ -49,37 +60,42 @@ shellInit:
 	.db	"Collapse OS", 0
 
 shellLoop:
-	; check if the input buffer is full or ends in CR or LF. If it does,
-	; prints it back and empty it.
-	call	aciaBufPtr
-	cp	0
-	jr	z, shellLoop	; BUFIDX is zero? nothing to check.
-
-	cp	ACIA_BUFSIZE
-	jr	z, .do		; if BUFIDX == BUFSIZE? do!
-
-	; our previous char is in BUFIDX - 1. Fetch this
-	dec	hl
-	ld	a, (hl)		; now, that's our char we have in A
-	inc	hl		; put HL back where it was
-
+	; First, let's wait until something is typed.
+	call	aciaGetC
+	; got it. Now, is it a CR or LF?
 	cp	ASCII_CR
 	jr	z, .do		; char is CR? do!
 	cp	ASCII_LF
 	jr	z, .do		; char is LF? do!
 
-	; nothing matched? don't do anything
-	jr	shellLoop
-.do:
-	; terminate our string with 0
+	; Ok, gotta add it do the buffer
+	; save char for later
+	ex	af, af'
+	ld	hl, SHELL_BUF
+	call	findnull	; HL points to where we need to write
+				; A is the number of chars in the buf
+	cp	SHELL_BUFSIZE
+	jr	z, .do		; A == bufsize? then our buffer is full. do!
+
+	; bring the char back in A
+	ex	af, af'
+	; Buffer not full, not CR or LF. Let's put that char in our buffer and
+	; read again.
+	ld	(hl), a
+	; Now, write a zero to the next byte to properly terminate our string.
+	inc	hl
 	xor	a
 	ld	(hl), a
-	; reset buffer index
-	ld	(ACIA_BUFIDX), a
 
-	; alright, let's go!
-	ld	hl, ACIA_BUF
+	jr	shellLoop
+
+.do:
+	ld	hl, SHELL_BUF
 	call	shellParse
+	; empty our buffer by writing a zero to its first char
+	xor	a
+	ld	(hl), a
+
 	jr	shellLoop
 
 printcrlf: