- Optimized LAN91C96 driver for speed and size.

- Speed: The primary byte copy loops are reduzed to the bare minimum by adjusting the base pointer 'ptr' and loop register 'y' in such a way that the 'y' overflow matches the low byte of the loop size.

- Introduced a loop for setting the MAC address.

Additional minor fix:

- Properly start self modification with first location.

cpu/6502/net/lan91c96.S

@@ -78,8 +78,7 @@ fixup:	.byte	fixup02-fixup01, fixup03-fixup02, fixup04-fixup03
 	.byte	fixup29-fixup28, fixup30-fixup29, fixup31-fixup30
 	.byte	fixup32-fixup31, fixup33-fixup32, fixup34-fixup33
 	.byte	fixup35-fixup34, fixup36-fixup35, fixup37-fixup36
-	.byte	fixup38-fixup37, fixup39-fixup38, fixup40-fixup39
+	.byte	fixup38-fixup37
-	.byte	fixup41-fixup40, fixup42-fixup41
 
 fixups	= * - fixup
 
@@ -160,14 +159,14 @@ init:
 
 	; Reset ETH card
 :	lda #$00		; Bank 0
-fixup00:sta ethbsr
+fixup01:sta ethbsr
 
 	lda #%10000000		; Software reset
-fixup01:sta ethrcr+1
+fixup02:sta ethrcr+1
 
 	ldy #$00
-fixup02:sty ethrcr
+fixup03:sty ethrcr
-fixup03:sty ethrcr+1
+fixup04:sty ethrcr+1
 
 	; Delay
 :	cmp ($FF,x)		; 6 cycles
@@ -179,45 +178,39 @@ fixup03:sty ethrcr+1
 	; Enable transmit and receive
 	lda #%10000001		; Enable transmit TXENA, PAD_EN
 	ldx #%00000011		; Enable receive, strip CRC ???
-fixup04:sta ethtcr
+fixup05:sta ethtcr
-fixup05:stx ethrcr+1
+fixup06:stx ethrcr+1
 
 	lda #$01		; Bank 1
-fixup06:sta ethbsr
+fixup07:sta ethbsr
 
-fixup07:lda ethcr+1
+fixup08:lda ethcr+1
 	ora #%00010000		; No wait (IOCHRDY)
-fixup08:sta ethcr+1
+fixup09:sta ethcr+1
 
 	lda #%00001001		; Auto release
-fixup09:sta ethctr+1
+fixup10:sta ethctr+1
 
 	; Set MAC address
-	lda mac
+	ldy #$00
-	ldx mac+1
+:	lda mac,y
-fixup10:sta ethiar
+fixup11:sta ethiar,y
-fixup11:stx ethiar+1
+	iny
-	lda mac+2
+	cpy #$06
-	ldx mac+3
+	bcc :-
-fixup12:sta ethiar+2
-fixup13:stx ethiar+3
-	lda mac+4
-	ldx mac+5
-fixup14:sta ethiar+4
-fixup15:stx ethiar+5
 
 	; Set interrupt mask
 	lda #$02		; Bank 2
-fixup16:sta ethbsr
+fixup12:sta ethbsr
 
 	lda #%00000000		; No interrupts
-fixup17:sta ethmsk
+fixup13:sta ethmsk
 	rts
 
 ;---------------------------------------------------------------------
 
 poll:
-fixup18:lda ethist
+fixup14:lda ethist
 	and #%00000001		; RCV INT
 	bne :+
 
@@ -230,12 +223,12 @@ fixup18:lda ethist
 	
 :	lda #$00
 	ldx #%11100000		; RCV, AUTO INCR., READ
-fixup19:sta ethptr
+fixup15:sta ethptr
-fixup20:stx ethptr+1
+fixup16:stx ethptr+1
 
 	; Last word contains 'last data byte' and $60 or 'fill byte' and $40
-fixup21:lda ethdata		; Status word
+fixup17:lda ethdata		; Status word
-fixup22:lda ethdata		; Need high byte only
+fixup18:lda ethdata		; Need high byte only
 
 	; Move ODDFRM bit into carry:
 	; - Even packet length -> carry clear -> subtract 6 bytes
@@ -247,10 +240,10 @@ fixup22:lda ethdata		; Need high byte only
 	lsr
 
 	; The packet contains 3 extra words
-fixup23:lda ethdata		; Total number of bytes
+fixup19:lda ethdata		; Total number of bytes
 	sbc #$05		; Actually 5 or 6 depending on carry
 	sta len
-fixup24:lda ethdata
+fixup20:lda ethdata
 	sbc #$00
 	sta len+1
 
@@ -265,7 +258,7 @@ fixup24:lda ethdata
 	; Yes, skip packet
 	; Remove and release RX packet from the FIFO
 	lda #%10000000
-fixup25:sta ethmmucr
+fixup21:sta ethmmucr
 
 	; No packet available
 	lda #$00
@@ -273,26 +266,19 @@ fixup25:sta ethmmucr
 	rts
 
 	; Read bytes into buffer
-:	lda bufaddr
+:	jsr adjustptr
-	ldx bufaddr+1
+:
-	sta ptr
+fixup22:lda ethdata
-	stx ptr+1
-	ldx len+1
-	ldy #$00
-read:
-fixup26:lda ethdata
 	sta (ptr),y
 	iny
-	bne :+
+	bne :-
 	inc ptr+1
-:	cpy len
-	bne read
 	dex
-	bpl read
+	bpl :-
 
 	; Remove and release RX packet from the FIFO
 	lda #%10000000
-fixup27:sta ethmmucr
+fixup23:sta ethmmucr
 
 	; Return packet length
 	lda len
@@ -309,14 +295,14 @@ send:
 	; Allocate memory for TX
 	txa
 	ora #%00100000
-fixup28:sta ethmmucr
+fixup24:sta ethmmucr
 
 	; 8 retries
 	ldy #$08
 
 	; Wait for allocation ready
 :
-fixup29:lda ethist
+fixup25:lda ethist
 	and #%00001000		; ALLOC INT
 	bne :+
 
@@ -330,21 +316,21 @@ fixup29:lda ethist
 
 	; Acknowledge interrupt, is it necessary ???
 :	lda #%00001000
-fixup30:sta ethack
+fixup26:sta ethack
 
 	; Set packet address
-fixup31:lda etharr
+fixup27:lda etharr
-fixup32:sta ethpnr
+fixup28:sta ethpnr
 
 	lda #$00
 	ldx #%01000000		; AUTO INCR.
-fixup33:sta ethptr
+fixup29:sta ethptr
-fixup34:stx ethptr+1
+fixup30:stx ethptr+1
 
 	; Status written by CSMA
 	lda #$00
-fixup35:sta ethdata
+fixup31:sta ethdata
-fixup36:sta ethdata
+fixup32:sta ethdata
 
 	; Check packet length parity:
 	; - Even packet length -> carry set   -> add 6 bytes
@@ -356,30 +342,23 @@ fixup36:sta ethdata
 	; The packet contains 3 extra words
 	lda len
 	adc #$05		; Actually 5 or 6 depending on carry
-fixup37:sta ethdata
+fixup33:sta ethdata
 	lda len+1
 	adc #$00
-fixup38:sta ethdata
+fixup34:sta ethdata
 
 	; Send the packet
 	; ---------------
 
 	; Write bytes from buffer
-	lda bufaddr
+	jsr adjustptr
-	ldx bufaddr+1
+:	lda (ptr),y
-	sta ptr
+fixup35:sta ethdata
-	stx ptr+1
-	ldx len+1
-	ldy #$00
-write:	lda (ptr),y
-fixup39:sta ethdata
 	iny
-	bne :+
+	bne :-
 	inc ptr+1
-:	cpy len
-	bne write
 	dex
-	bpl write
+	bpl :-
 
 	; Odd packet length ?
 	lda len
@@ -392,13 +371,13 @@ fixup39:sta ethdata
 
 	; No
 :	lda #$00
-fixup40:sta ethdata		; Fill byte
+fixup36:sta ethdata		; Fill byte
 :	
-fixup41:sta ethdata		; Control byte
+fixup37:sta ethdata		; Control byte
 
 	; Add packet to FIFO
 	lda #%11000000		; ENQUEUE PACKET - transmit packet
-fixup42:sta ethmmucr
+fixup38:sta ethmmucr
 	rts
 
 ;---------------------------------------------------------------------
@@ -407,3 +386,21 @@ exit:
 	rts
 
 ;---------------------------------------------------------------------
+
+adjustptr:
+	lda len
+	eor #$FF		; Two's complement part 1
+	tay
+	iny			; Two's complement part 2
+	sty reg
+	sec
+	lda bufaddr
+	sbc reg
+	sta ptr
+	lda bufaddr+1
+	sbc #$00
+	sta ptr+1
+	ldx len+1
+	rts
+
+;---------------------------------------------------------------------