Date   : Sat, 16 Feb 2008 00:31:39 +0000 (GMT)
From   : tommowalker@... (Tom Walker)
Subject: BBC FPGA Boots to BASIC... almost...

> Actually, the 6502 will read anything placed on the bus by any device which
> responds to the read. The usual case is there's nothing responding and
> driving the bus at this time (unless you've got some faulty or badly
> designed hardware), so the bus will return &FF (all high), not the previous
> value placed there.
> 
> If the address is within the valid address range of some hardware device
> but
> not a valid port/register, it will be up to the device itself what gets
> placed on the bus. (eg. most CRTC's will return 0 for out of range
> registers).
> 
> &FE never gets placed on the data bus anyway, it gets placed on the upper 8
> address lines of the CPU.

It gets read off the data bus by the CPU in the previous cycle, as it's
(usually) the last operand byte of whatever instruction is reading from IO
space.

Tom


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