Apple II System Monitor

Apple II System Monitor Commands

Quick Navigation

Entering the Monitor
Basic Commands
Memory Examination
Memory Modification
Execution Commands
Arithmetic Operations
Move & Verify
Register Commands
I/O Commands
Mini-Assembler
SWEET16
ROM Routines
Important Vectors
Tips & Tricks

Entering the Monitor

Method	Command	Description
From BASIC	CALL -151	Enter monitor from Applesoft or Integer BASIC
From DOS	CALL -151	Same as BASIC
Reset + Key	RESET then CTRL+B	On older Apple IIs
From Assembly	JMP $FF69	Jump to monitor entry point
Break	BRK	Execute BRK instruction (if vector set)
Power On	*	Some Apple II models boot to monitor

The monitor prompt is an asterisk (*)

Basic Commands

Command	Syntax	Description	Example
Exit Monitor	CTRL+C or CTRL+B	Return to BASIC	^C
Exit to Integer BASIC	E000G	Cold start Integer BASIC	*E000G
Exit to Applesoft	E003G	Cold start Applesoft	*E003G
Warm Start DOS	3D0G	Return to DOS without destroying program	*3D0G
Cold Start DOS	3D3G	Restart DOS (clears program)	*3D3G
Reset	CTRL+RESET	Hardware reset	-

Memory Examination Commands

Command	Syntax	Description	Example
Display Byte	addr	Display single byte	*300 0300- A9
Display Range	addr.addr	Display memory range	*300.31F 0300- A9 00 85...
Display Next	(just press Return)	Display next 8 bytes	* 0308- 00 00 00...
Display Page	addr.addr	Display full page	*300.3FF
ASCII Display	(automatic)	Shows ASCII on right side	*300.30F 0300- C1 D0 D0 CC C5 APPLE

Display Format

*800.807 0800- A9 00 AA 20 EF FF E8 60 .. ....` ↑ ↑ ↑ Address Hex bytes ASCII

Memory Modification Commands

Command	Syntax	Description	Example
Store Byte	addr:byte	Store single byte	*300:A9
Store Multiple	addr:b1 b2 b3...	Store multiple bytes	*300:A9 00 85 06
Modify Mode	addr:byteReturn	Enter modify mode	300:A9 301:_
Previous in Modify	← or -	Go back one byte	301:← 300:A9
Next in Modify	→ or Space	Go forward one byte	300:A9 Space 301:_
Exit Modify	Return	Exit modify mode	301:Return
ASCII Entry	addr:"text"	Enter ASCII text (with high bit set)	*300:"HELLO"

Use period (.) to separate addresses in ranges, colon (:) to store data

Execution Commands

Command	Syntax	Description	Example
Go/Run	addrG	Execute from address	*300G
Jump	addrJ	JMP to address	*300J
Trace	addrT	Single-step execution	*300T
Step	addrS	Step over subroutine	*300S
Continue	CTRL+Y	Continue from breakpoint	^Y
Call Subroutine	addrCTRL+E	JSR to address	*FD8E^E

Execution Notes

G - Starts execution and doesn't return unless program does RTS to monitor
J - Same as G but displays registers first
T - Executes one instruction and shows registers (trace mode)
S - Steps over JSR calls (executes whole subroutine)

Arithmetic Operations

Command	Syntax	Description	Example
Add	addr+addr	Add two addresses	*1000+80 1080
Subtract	addr-addr	Subtract addresses	*1000-80 0F80
Hex to Dec	=addr	Convert hex to decimal	*=FF 255
Dec to Hex	=+decimal	Convert decimal to hex	*=+255 00FF
Two's Complement	=-addr	Calculate two's complement	*=-1 FFFF

The monitor uses 16-bit arithmetic for all calculations

Move & Verify Commands

Command	Syntax	Description	Example
Move Memory	dest<start.endM	Move/copy memory block	*4000<300.3FFM
Verify	dest<start.endV	Compare memory blocks	*4000<300.3FFV
Fill Memory	start.end:byte	Fill range with byte	*300.3FF:00
Search Memory	start<byte byte...S	Search for byte pattern	*300<A9 00S

Move Command Details

*4000<300.3FFM ; Copy $300-$3FF to $4000-$40FF *800<300.3FFM ; Copy $300-$3FF to $800-$8FF Note: Source and destination can overlap

Register Display & Modification

Command	Syntax	Description	Example
Display All	CTRL+E	Display all registers	*^E A=00 X=00 Y=00 P=30 S=F8
Set Accumulator	A:value	Set A register	*A:FF
Set X Register	X:value	Set X register	*X:01
Set Y Register	Y:value	Set Y register	*Y:80
Set Status	P:value	Set processor status	*P:30
Set Stack Ptr	S:value	Set stack pointer	*S:FF

Status Register Flags

Bit 7

N - Negative

Bit 6

V - Overflow

Bit 5

- - Unused (always 1)

Bit 4

B - Break

Bit 3

D - Decimal

Bit 2

I - Interrupt Disable

Bit 1

Z - Zero

Bit 0

C - Carry

Input/Output Commands

Command	Syntax	Description	Example
Input from Keyboard	I	Input hex byte from keyboard	*300:I (type two hex digits)
Output to Screen	O	Output ASCII character	*300:41 O A
Control Output	CTRL+K	Set output hook	*^K
Normal Output	N	Reset to normal I/O	*N
User Vector	addrU	Set user command vector	*300U

Mini-Assembler (! Command)

Command	Description	Example
!	Enter mini-assembler	*! !_
addr:opcode	Assemble instruction	!300:LDA #$00
Return	Next address	!302:_
$	Exit assembler	!$ *

Mini-Assembler Examples

*! ; Enter mini-assembler !300:LDA #$00 ; A9 00 !STA $06 ; 85 06 (auto-increments) !INX ; E8 !RTS ; 60 !$ ; Exit to monitor *300L ; Disassemble to verify 0300- A9 00 LDA #$00 0302- 85 06 STA $06 0304- E8 INX 0305- 60 RTS

The mini-assembler supports standard 6502 mnemonics and addressing modes

SWEET16 Commands

SWEET16 is a 16-bit virtual machine interpreter built into Apple II ROM

To Use SWEET16	Description
300:20 00 F8	JSR $F800 - Enter SWEET16
303:(SWEET16 code)	Place SWEET16 opcodes
xxx:00	RTN - Return to 6502

SWEET16 Registers

R0-R15: Sixteen 16-bit registers
R0: Accumulator
R12: Subroutine stack
R13: Comparison results
R14: Status register
R15: Program counter

Useful ROM Monitor Routines

Address	Name	Function	Registers Used
$F800	SWEET16	Enter SWEET16 interpreter	All
$F819	SET	Set SWEET16 register	-
$F831	LD	Load SWEET16 register	-
$F941	PRNTAX	Print A and X in hex	A, X
$F944	PRBLNK	Print 3 spaces	X=3
$F948	PRBL2	Print X spaces	X
$FA62	RTS1	Do RTS	-
$FAA6	OLDRST	Old reset entry	All
$FB1E	PREAD	Read paddle	X
$FB2F	INIT	Initialize text screen	A
$FB39	SETTXT	Set text mode	-
$FB40	SETGR	Set graphics mode	-
$FB60	SCRN	Read text screen	A, X, Y
$FBC1	BASCALC	Calculate text base address	A
$FBD9	BELL1	Ring bell	A
$FBE4	BELL2	Ring bell twice	A
$FC22	VTAB	Vertical tab to A	A
$FC24	VTABZ	Vertical tab to CV	-
$FC42	CLREOP	Clear to end of page	Y
$FC58	HOME	Clear screen	A, Y
$FC62	CR	Carriage return	A
$FC66	LF	Line feed	A
$FC9C	CLREOL	Clear to end of line	Y
$FCA8	WAIT	Delay 1/2(26+27A+5A²) μs	A
$FD0C	RDKEY	Read keyboard	A
$FD1B	KEYIN	Read keyboard (no cursor)	A
$FD35	RDCHAR	Read char with cursor	A
$FD6A	GETLN	Get line of input	A, X
$FD6F	GETLNZ	Get line (use prompt)	A, X
$FD8E	CROUT	Output carriage return	A
$FDA3	PRBYTE	Print byte in hex	A
$FDDA	PRHEX	Print nibble in hex	A
$FDE3	PRNTYX	Print Y and X in hex	Y, X
$FDED	COUT	Output character	A
$FDF0	COUT1	Output to screen	A
$FE2C	MOVE	Move memory	A, Y
$FE5E	LIST	Disassembler	A, X, Y
$FE80	SETINV	Set inverse text	-
$FE84	SETNORM	Set normal text	-
$FEB0	SETVID	Set video mode	-
$FEC5	WRITE	Write cassette tape	A, Y
$FECC	READ	Read cassette tape	A, Y
$FF3A	BELL	Ring bell	-
$FF59	BRKVEC	BRK vector location	-
$FF65	MON	Enter monitor	All
$FF69	MONZ	Enter monitor quietly	All
$FF70	TRACE	Trace one instruction	All

Important Memory Locations & Vectors

Address	Name	Description
$0020-$002F	WNDLFT/TOP/WIDTH/BTM	Text window parameters
$0024	CH	Horizontal cursor position
$0025	CV	Vertical cursor position
$0026-$0027	GBASL/H	Graphics base address
$0028-$0029	BASL/H	Text base address
$0033	PROMPT	Monitor prompt character
$0036-$0037	CSW	Character output hook
$0038-$0039	KSW	Keyboard input hook
$003A-$003B	PCL/H	Program counter storage
$003C-$003D	A1L/H	Monitor work area
$003E-$003F	A2L/H	Monitor work area
$0040-$0041	A3L/H	Monitor work area
$0042-$0043	A4L/H	Monitor work area
$0045	ACC	Accumulator storage
$0046	XREG	X register storage
$0047	YREG	Y register storage
$0048	STATUS	Status register storage
$0049	SPNT	Stack pointer storage
$0050-$00FF	STACK	6502 stack area (grows down from $01FF)
$0200-$02FF	INBUF	Input buffer
$03F0-$03F1	BRKV	BRK vector (normally $FA59)
$03F2-$03F3	SOFTEV	Soft entry vector (normally $FF59)
$03F4	PWREDUP	Power-up byte
$03F5-$03F7	USRV	User command vector (JMP)
$03F8-$03FA	NMI	NMI vector (JMP)
$03FB-$03FD	IRQV	IRQ vector (JMP)
$FFFA-$FFFB	NMIVEC	NMI hardware vector
$FFFC-$FFFD	RESETV	RESET hardware vector
$FFFE-$FFFF	IRQVEC	IRQ hardware vector

Monitor Tips & Tricks

Quick Disassembly: Use addrL to disassemble 20 instructions starting at addr

Memory Search: To find a byte sequence, use the Search command: *800<A9 00 85S

Breakpoints: Replace an instruction with BRK ($00) to create a breakpoint. Original byte saved at $49

Quick Fill: Fill memory with zeros: *300.3FF:0

Save Typing: After entering an address once, subsequent commands use it as default

ASCII Text: View memory as ASCII by looking at the right side of the display

Register Shortcut: Just press ^E to see all registers quickly

Modify Memory: Use Space to advance, minus to go back while modifying

Hex Math: Monitor supports 16-bit addition and subtraction directly

Call ROM: Many ROM routines can be called directly, e.g., *FC58G to clear screen

Common Monitor Workflows

1. Write and test a small routine: *300:A9 00 ; LDA #$00 *302:85 06 ; STA $06 *304:60 ; RTS *300L ; Disassemble to verify *300G ; Run it 2. Save memory to cassette (old school!): *800.8FFW ; Write $800-$8FF to tape *800.8FFR ; Read it back 3. Examine and modify a BASIC program: *67.68 ; See program start pointer *800.9FF ; View program area 4. Debug with breakpoints: *305 ; See what's at $305 *305:00 ; Replace with BRK *300G ; Run until breakpoint *^E ; Check registers *305:XX ; Restore original byte *^Y ; Continue execution

Historical Note: The Apple II System Monitor, written by Steve Wozniak, fit in just 2KB of ROM and provided a complete development environment. Its compact design and powerful features made it legendary among early microcomputer monitors.