6502 Startup Process

[Scott Alfter]

I have a 6502 lying around now from when I put an enhancement kit in my IIe,
and I was thinking of possibly putting it to use outside the Apple--maybe as
a calculator that does logic functions or something. (It would be handy for
someone like me who does lots of machine-level work, and no, a 6502 would not
be improperly used in a calculator--look at what Hewlett-Packard does with
the 6502 and (more recently) the 65816.) There's only one problem I can think
of--how does the thing start up?

Every day for the past five years I've taken the 6502's startup process pretty
much for granted when it throws "Apple ][" (or more recently, "Apple //e")
across the top of the monitor and starts loading a DOS in from the drive. How
does the processor get to that point, though? I know what ROM routines are
responsible for the boot process, but how does the 65(C)02 know where those
routines are, and how does it call them? I don't think this is something the
_Apple_IIe_Technical_Reference_Manual_ covers, as it is not a IIe-specific
thing, but is related more to the processor than anything else.

Scott Alfter-------------------------------------------------------------------
Bitnet: free0066@uiucvmd _/_ Apple II: the power to be your best!
Internet: alf...@mrcnext.cso.uiuc.edu / v
cs1...@ux1.cso.uiuc.edu ( ( A keyboard--how quaint!
saa3...@uxa.cso.uiuc.edu _^_/ --M. Scott, STIV

[wogg...@uxa.cso.uiuc.edu]

You have to rig it so when the power comes on, the processor gets a
RESET (you bring high the reset line, I think). You could do this
simply by providing yourself with a button that completes a circuit
(thus giving you a reset button into the bargain, but also making this
part of the manditory start-up process) or else rig a more complicated
circuit to do this once ever time the power comes on (like Apple uses)

The other half of it is that you have to use a ROM at the top of memory
to hold the interupt vectors (including the one for reset). Point the
reset vector to your startup code.

[Stephen Brown]

In-Reply-To: message from FREE...@VMD.CSO.UIUC.EDU

Scott Alfter asks how the 6502 starts up. It starts up via the RESET vector at
$FFFC, $FFFD. I believe the 65816 powers up in Emulation mode, so you'd find
its reset vector at 00/FFFC, 00/FFFD. Note that the CPU has to be reset when
its turned on, to put the processor into a known state.

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