Woz Monitor ASM

SOURCE: http://www.sbprojects.com/projects/apple1/wozmon.asm

;-------------------------------------------------------------------------
;
; The WOZ Monitor for the Apple 1
; Written by Steve Wozniak 1976
;
;-------------------------------------------------------------------------

.CR 6502
.OR $FF00
.TF WOZMON.HEX,HEX,8

;-------------------------------------------------------------------------
; Memory declaration
;-------------------------------------------------------------------------

XAML .EQ $24 Last "opened" location Low
XAMH .EQ $25 Last "opened" location High
STL .EQ $26 Store address Low
STH .EQ $27 Store address High
L .EQ $28 Hex value parsing Low
H .EQ $29 Hex value parsing High
YSAV .EQ $2A Used to see if hex value is given
MODE .EQ $2B $00=XAM, $7F=STOR, $AE=BLOCK XAM

IN .EQ $0200,$027F Input buffer

KBD .EQ $D010 PIA.A keyboard input
KBDCR .EQ $D011 PIA.A keyboard control register
DSP .EQ $D012 PIA.B display output register
DSPCR .EQ $D013 PIA.B display control register

; KBD b7..b0 are inputs, b6..b0 is ASCII input, b7 is constant high
; Programmed to respond to low to high KBD strobe
; DSP b6..b0 are outputs, b7 is input
; CB2 goes low when data is written, returns high when CB1 goes high
; Interrupts are enabled, though not used. KBD can be jumpered to IRQ,
; whereas DSP can be jumpered to NMI.

;-------------------------------------------------------------------------
; Constants
;-------------------------------------------------------------------------

BS .EQ $DF Backspace key, arrow left key
CR .EQ $8D Carriage Return
ESC .EQ $9B ESC key
PROMPT .EQ "\" Prompt character

;-------------------------------------------------------------------------
; Let's get started
;
; Remark the RESET routine is only to be entered by asserting the RESET
; line of the system. This ensures that the data direction registers
; are selected.
;-------------------------------------------------------------------------

RESET CLD Clear decimal arithmetic mode
CLI
LDY #%0111.1111 Mask for DSP data direction reg
STY DSP (DDR mode is assumed after reset)
LDA #%1010.0111 KBD and DSP control register mask
STA KBDCR Enable interrupts, set CA1, CB1 for
STA DSPCR positive edge sense/output mode.

; Program falls through to the GETLINE routine to save some program bytes
; Please note that Y still holds $7F, which will cause an automatic Escape

;-------------------------------------------------------------------------
; The GETLINE process
;-------------------------------------------------------------------------

NOTCR CMP #BS Backspace key?
BEQ BACKSPACE Yes
CMP #ESC ESC?
BEQ ESCAPE Yes
INY Advance text index
BPL NEXTCHAR Auto ESC if line longer than 127

ESCAPE LDA #PROMPT Print prompt character
JSR ECHO Output it.

GETLINE LDA #CR Send CR
JSR ECHO

LDY #0+1 Start a new input line
BACKSPACE DEY Backup text index
BMI GETLINE Oops, line's empty, reinitialize

NEXTCHAR LDA KBDCR Wait for key press
BPL NEXTCHAR No key yet!
LDA KBD Load character. B7 should be '1'
STA IN,Y Add to text buffer
JSR ECHO Display character
CMP #CR
BNE NOTCR It's not CR!

; Line received, now let's parse it

LDY #-1 Reset text index
LDA #0 Default mode is XAM
TAX X=0

SETSTOR ASL Leaves $7B if setting STOR mode

SETMODE STA MODE Set mode flags

BLSKIP INY Advance text index

NEXTITEM LDA IN,Y Get character
CMP #CR
BEQ GETLINE We're done if it's CR!
CMP #"."
BCC BLSKIP Ignore everything below "."!
BEQ SETMODE Set BLOCK XAM mode ("." = $AE)
CMP #":"
BEQ SETSTOR Set STOR mode! $BA will become $7B
CMP #"R"
BEQ RUN Run the program! Forget the rest
STX L Clear input value (X=0)
STX H
STY YSAV Save Y for comparison

; Here we're trying to parse a new hex value

NEXTHEX LDA IN,Y Get character for hex test
EOR #$B0 Map digits to 0-9
CMP #9+1 Is it a decimal digit?
BCC DIG Yes!
ADC #$88 Map letter "A"-"F" to $FA-FF
CMP #$FA Hex letter?
BCC NOTHEX No! Character not hex

DIG ASL
ASL Hex digit to MSD of A
ASL
ASL

LDX #4 Shift count
HEXSHIFT ASL Hex digit left, MSB to carry
ROL L Rotate into LSD
ROL H Rotate into MSD's
DEX Done 4 shifts?
BNE HEXSHIFT No, loop
INY Advance text index
BNE NEXTHEX Always taken

NOTHEX CPY YSAV Was at least 1 hex digit given?
BEQ ESCAPE No! Ignore all, start from scratch

BIT MODE Test MODE byte
BVC NOTSTOR B6=0 is STOR, 1 is XAM or BLOCK XAM

; STOR mode, save LSD of new hex byte

LDA L LSD's of hex data
STA (STL,X) Store current 'store index'(X=0)
INC STL Increment store index.
BNE NEXTITEM No carry!
INC STH Add carry to 'store index' high
TONEXTITEM JMP NEXTITEM Get next command item.

;-------------------------------------------------------------------------
; RUN user's program from last opened location
;-------------------------------------------------------------------------

RUN JMP (XAML) Run user's program

;-------------------------------------------------------------------------
; We're not in Store mode
;-------------------------------------------------------------------------

NOTSTOR BMI XAMNEXT B7 = 0 for XAM, 1 for BLOCK XAM

; We're in XAM mode now

LDX #2 Copy 2 bytes
SETADR LDA L-1,X Copy hex data to
STA STL-1,X 'store index'
STA XAML-1,X and to 'XAM index'
DEX Next of 2 bytes
BNE SETADR Loop unless X = 0

; Print address and data from this address, fall through next BNE.

NXTPRNT BNE PRDATA NE means no address to print
LDA #CR Print CR first
JSR ECHO
LDA XAMH Output high-order byte of address
JSR PRBYTE
LDA XAML Output low-order byte of address
JSR PRBYTE
LDA #":" Print colon
JSR ECHO

PRDATA LDA #" " Print space
JSR ECHO
LDA (XAML,X) Get data from address (X=0)
JSR PRBYTE Output it in hex format
XAMNEXT STX MODE 0 -> MODE (XAM mode).
LDA XAML See if there's more to print
CMP L
LDA XAMH
SBC H
BCS TONEXTITEM Not less! No more data to output

INC XAML Increment 'examine index'
BNE MOD8CHK No carry!
INC XAMH

MOD8CHK LDA XAML If address MOD 8 = 0 start new line
AND #00.0111
BPL NXTPRNT Always taken.

;-------------------------------------------------------------------------
; Subroutine to print a byte in A in hex form (destructive)
;-------------------------------------------------------------------------

PRBYTE PHA Save A for LSD
LSR
LSR
LSR MSD to LSD position
LSR
JSR PRHEX Output hex digit
PLA Restore A

; Fall through to print hex routine

;-------------------------------------------------------------------------
; Subroutine to print a hexadecimal digit
;-------------------------------------------------------------------------

PRHEX AND #00.1111 Mask LSD for hex print
ORA #"0" Add "0"
CMP #"9"+1 Is it a decimal digit?
BCC ECHO Yes! output it
ADC #6 Add offset for letter A-F

; Fall through to print routine

;-------------------------------------------------------------------------
; Subroutine to print a character to the terminal
;-------------------------------------------------------------------------

ECHO BIT DSP DA bit (B7) cleared yet?
BMI ECHO No! Wait for display ready
STA DSP Output character. Sets DA
RTS

;-------------------------------------------------------------------------
; Vector area
;-------------------------------------------------------------------------

.DA $0000 Unused, what a pity
NMI_VEC .DA $0F00 NMI vector
RESET_VEC .DA RESET RESET vector
IRQ_VEC .DA $0000 IRQ vector

;-------------------------------------------------------------------------

.LI OFF