Apple II Monitor usage

Apple II series ROM contains a simple but powerful machine level monitor to display and modify machine's memory. While original Apple II actually starts up directly in the monitor, later Apple models automatically boot into the BASIC program.

Monitor also includes a mini-assembler, a very rudimentary assembler for 6502.

Entering and exiting the Monitor

Entering the Monitor from BASIC can be done by executing CALL -151:

] CALL -151

*

Exiting the monitor can be accomplished in multiple ways:

Control-B (and ENTER) - will do a cold start (jumps to address $E000)
Control-Y (and ENTER) - will do a warm start (jumps to address $E003)

Mini-Assembler

While Mini-assembler seems to be included with all Apple II models except Apple II+, invoking it depends on the machine type:

Apple //e: Just enter ! in the monitor prompt
Others: Run F666G in the monitor prompt

Once in the mini-assembler, the prompt should change to ! and you can start entering the instructions.

To test this out, here's a small program that prints "APPLE" on the screen. Note that all the entry lines start with !-prompt:

! 300: JSR FC58
0300-   20 58 FC    JSR $FC58
! LDA #C1
0303-   A9 C1       LDA #$C1
! STA 700
0305-   8D 00 07    STA $0700
! LDA #D0
0308-   A9 D0       LDA #$D0
! STA 701
030A-   8D 01 70    STA $0701
! STA 702
030D-   8D 02 70    STA $0702
! LDA #CC
0310-   A9 CC       LDA #$CC
! STA 703
0312-   8D 03 70    STA $0703
! LDA #C5
0315-   A9 C5       LDA #$C5
! STA 704
0317-   8D 04 70    STA $0704
! RTS
031A-   60          RTS

Now that program has been inserted, exit by just pressing enter in the prompt. In the monitor, one can inspect the code using the L (list) command by prepending the address to it:

* 300L

0300    20 58 FC    JSR $FC58
0303    A9 C1       LDA #$C1
0305    8D 00 07    STA $0700
0308    A9 D0       LDA #$D0
030A    8D 01 70    STA $0701
030D    8D 02 70    STA $0702
0310    A9 CC       LDA #$CC
0312    8D 03 70    STA $0703
0315    A9 C5       LDA #$C5
0317    8D 04 70    STA $0704
031A    60          RTS
...

And to run this, use the G (Go) command:

* 300G

If everything was typed in correctly, the screen should clear and you'll end up with APPLE written on sreen.

References

I have mostly used Roger Wagner's book "Assembly Lines: The Complete Book" to get myself started. It's also available on the Archive.org.