Re: Update on Apple II rev 0 replica kits

["Bryan Parkoff" <nospam@nospam.com>]

Mike,

> Power up without reset results in a random text filled screen and no 
> prompt.  Actual display depends on what state memory comes up in.  My 
> experience is that the screen is mostly filled with the same inverse 
> character, with a few oddballs spotted around.  I am unsure of what the 
> processor is doing - it would take a logic analyzer to determine if it 
> normally processes instructions, without a proper reset, or just hangs. I 
> suspect you could simulate the display simply by holding down reset when 
> powering up a later rev board.

    I am surprised.  It can't be right for rev 0.  After power-up, random 16 
colored LO-RES blocks should be displayed in the screen.  Why?  All 
softswitches are always turned off (or all individual softswitch's bit is 
zero) after power-up.  It is the same example.  The internal registers like 
Accumulator, Index X, Y, Stack Pointer, and PC inside MPU are always set to 
zero after power-up.  It is possible that MPU continues to process the first 
instruction at PC = $0000.  It may process BRK, push low/high PC and flags 
into stack and pop them out of stack at endless loop.  Nothing changes.  It 
may be possible that zero page might contain random HEX code after power up. 
I suppose that all RAM chips should always fill "FF" or "00" in all $00 page 
through $BF page.
    After you hit RESET, MPU starts to read $FFFC - $FFFD and store two 
bytes into low / high PC.  Then, the start-up routine at $FF59 initializes 
to turn off LO-RES and turn on TEXT and fill information into zero page. 
Monitor prompt is ready for you to type.  Surprise?  The start-up routine 
does not set $FF to stack pointer.  Low PC is pushed into stack -- $0100. 
The stack pointer decrements from $00 to $FF.  High PC is pushed into 
stack -- $01FF.  The stack pointer decrements from $FF to $FE.

Bryan Parkoff