wozmon is a tiny but elegant monitor program Steve Wozniak wrote for the original Apple-1 computer in 1976. It lets you examine memory, change memory, and run machine-language programs directly from the keyboard. This tutorial is designed to teach you how to use HEXMON, which is a very similar program.

All addresses and data values are in hexadecimal (no $ prefix needed). Press RETURN (or Enter) after each line to execute it. The prompt for the Apple-1 was a backslash \, sometimes shown as a @ in emulators/replicas. The prompt was changed to a * for the Apple II and that is the convention that HEXMON uses.

• Power on the SD-8516 / VC-3.
• After you hear the chime, you can type MON.
• You should see *. The system is now ready for input.

Single location: Type the 4-digit address and press RETURN Example:

  0030

This might respond 0030: A9 i.e. it shows the current byte at address $0030.

Range of locations: Use the dot operator . between start and end address.

Example:

  200.2FF

This dumps bytes from $0200 to $02FF, 8 bytes per line with address prefix and printable display.

Use the entry operator : after an address, followed by data bytes. Bytes are stored starting at the location you have specified.

Write-One

Example:

  1000:A9

This will store $A9 into memory location $1000.

Write-List Example:

  200: A9 00 20 EF FF

Stores those 5 bytes starting at address $0200.

Type the start address followed by R (Run).

Example:

  1000R

This will call the program at $1000 and begin executing code. If the program issues a RET, it will return to HEXMON. You could then quit HEXMON normally by typing Q.

It's worth noting you don't need HEXMON to run a program. If a program is assembled starting at $030100, typing SYS by itself on a line will run it. Otherwise you can type SYS $(address) at the command prompt to run a machine language program.

Example “Hello World” program:

  C000: 00 34 10 C0 00 00 20 66
  C008: 86 05 00 20 64 86 05 85
  C010: 48 45 4C 4C 4F 20 57 4F
  C018: 52 4C 44 21 00 00 00 00

Type this in and then type C000R to run the program!

You can also enter code from full disassembly listings. Just type in the values on the left, as shown above in 'example program'. The other parts of the disassembly are just for your reference and to help you learn what the program does.

 ADDR    BYTES          INSTRUCTION          COMMENT
--------------------------------------------------------------------------------
$C000:   00 34          LDBLX @msg           ; Load pointer to string into BLX
         10 C0 00                            ; (Bank 0, address $C0 10)
$C005:   00 20 66       LDAH $66             ; IO_PRINT_STR function
$C008:   86 05          INT $05              ; BASIC services library
$C00A:   00 20 64       LDAH $64             ; IO_NEWLINE function
$C00D:   86 05          INT $05              ; Call BASIC services
$C00F:   85             RET                  ; Return to caller
$C010:                  msg:                 ; String data label (equates to $C000 above)
$C010:   48 45 4C 4C    .bytes "HELLO        ; H E L L
$C014:   4F 20 57 4F           WORLD!", 0    ; O   W O
$C018:   52 4C 44 21                         ; R L D !
$C01C:   00                                  ; null terminator (zero terminated string)

Just like before, you can run the program by typing in C000R (run program at start location C000).

The RET ($85) at $C00F will return control to the monitor program.

HEXMON is equipped with three commands that help you load, save and publish machine language programs.

The L and S commands deal with binary data. The P command will produce a publishable listing like this:

  $C000: 00 34 10 C0 00 00 20 66 :8A
  $C008: 86 05 00 20 64 86 05 85 :1F
  $C010: 48 45 4C 4C 4F 20 57 4F :3A
  $C018: 52 4C 44 21 00 00 00 00 :03
  $C020: 00 :00

When you enter the program, you can skip the $ and the :byte checksum, or you can type them in (your choice). You can also substitute a comma , in place of a colon for the checksum.

You can also enter data like this:

  C000:003410C000002066

However, doing it this way will not verify the checksum. If you enter the program incorrectly there will be no way for you to automatically find the error.

Now you have everything you need to know to use HEXMON!

Many people still use it today because of its simplicity and elegance.

If you want to have some fun and try more example programs that can be loaded via HEXMON, please see: Assembly Programs.

If you are looking for a tutorial on how to write Assembly Language programs yourself, you can refer to:

• SD-8516 Assembly Language (crash course)
• SD-8516 Programmer's Reference Guide (full tutorial + resources)

Have fun exploring SD-8516 machine code!