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mg_notes:iie_card:weird_stuff [2017/08/07 22:16]
M.G. [Other Weird Instructions]
mg_notes:iie_card:weird_stuff [2017/08/07 22:25] (current)
M.G. Slot scan/select description.
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 Hit the left arrow a bunch of times until the display scrolls. **POOF!** Hit the left arrow a bunch of times until the display scrolls. **POOF!**
- + 
 +==== Slot Scan Scam ==== 
 + 
 +The %%//%%e Card lets the user pick the startup slot in the control panel or "​Scan"​ which is the behavior of a standard %%//%%e. 
 + 
 +This is implemented by the sequences $02 $02 which replaces the LDA #$C8 at the start of the slot scan loop, and $02 $03 which replaces the CMP #$C0 instruction that decides loop termination. 
 + 
 +The $02 $02 sequence loads the accumulator with $C8 if scan is selected, or $Cn+1 if a specific slot is selected. 
 + 
 +<​code>​ 
 +*300:02 02 4C DA FD 
 +*300G 
 +C8   (if scan or slot 7 selected, "​Cx"​ if another slot is selected) 
 +
 +</​code>​ 
 + 
 +The $02 $03 sequence behaves as if CMP #$C0 or CMP #$Cn-1 has been executed and if it has, displays "​UNABLE TO BOOT FROM STARTUP SLOT" in the center of the screen in a similar manner to the copyright message. ​ The message is not in Apple II memory. ​ It returns with the flags set as executing the CMP instruction would have. 
 +