S.Elliott's blog

Using the "Phi1 Echo" to distinguish disk controllers

Submitted by S.Elliott on July 1, 2023 - 5:49pm

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Apple hid a quirky feature inside its disk drives, a full-loop signal path via addressable latch Q1, via stepper phase-1, via the motor control board, via the write-protect switch, via the analog board in the disk drive, via the shift-right input into the data register, and back onto the bus.  This signal path can be used to distinguish between the various Apple II disk controllers.
  • Apple Disk II controller prints 2 inverse bytes: 7F 00
  • Micro Sci A2 controller prints 2 inverse bytes: 80 00
  • (untested guess) IWM in Apple //c: A0 20
  • (untested guess) IWM in Apple //c+: AF 2F [EDIT: this guess was wrong]
Tags: 
hardware

Quick ref: Pass arguments to machine code from the Monitor

Submitted by S.Elliott on April 28, 2023 - 6:18pm
When calling a machine code subroutine from the Monitor, one or two additional arguments can be passed by taking advantage of the way the Monitor uses delimiters to parse arguments from its command-line and stores them in the zero-page pointers A1, A2, A3, A4...in an arbitrarily-strange order:
  • The default argument is always the subroutine address, stored in A1.
  • Appending a '.' delimiter allows another argument to be stored in A2.
  • Inserting a '<' delimiter allows an argument to be stored in A4.
Tags: 
monitor
asm

Apple Presents...Easter Egg!

Submitted by S.Elliott on April 5, 2023 - 5:40pm
Back in 1983, how many users found the secret message in this instructional software?

]BRUN BARGLE

Bargle program by Sue Espinosa

Bargle Bargle Bargle

]

Tags: 
easter egg

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