25 Aug
2014
25 Aug
'14
12:48 p.m.
I just realized something, about the flashing LEDs..
They're not just flashing, I think the thing is +running+ or at least
counting. It's counting up through the entire 16-bit address space, and
displaying the position on the A15-A0 LEDs. Since A15 is the MSB, it
changes the least often, with A14 changing twice as often as A15, and A13
changing twice as often as A14 and so on down the line. Any bits below A13
or so are changing so quickly that vision can't perceive it.
So that is what I +think+ it's doing. As to why, I'd have little idea..
time to read more docs.
Are you sure it's counting up and not down?
AS I understnad it you have no memeory boards in the machine. The CPU (if
it's runnign at all) will be fetch instructions from a floating data bus.
I am assuming that it's an *080 or Z80 CPU/.
IF that floating bus is all 0's, then ti wil lbe fethcin NOPs and will be
counting up through memory, incrementign the pC after each NOP and
fetchign the next intruction form the next location.
But it's more likely (espeically grom what you have said) that it is
fetching FF's (all data lines are 1's). That's a RST 38h instruction.
Effectively iy dfoes a clal to lcation 0038 hex. Of course when it gts
there it will fetch another RST 38h from location 0038h, and carry on
doing the same thing. But it will also push the return address (when it is
fetcvhing a RST 38h from 0038h, this will mean it will write 39 00 to
memory) onto the stack. The stack builds downwards, so the address bus will
effectively alternate between 0038h (to fetch the isntruction) and a
downwards count (as it pushes the return address onto the stack). This
was actually a test ofn the TRS80 M1. You pulled the BASIC ROMs (causign
the CPU to read FF's from the data bus) and of course all memeory,
including the video RAM was fileld with 39 00 patters. Thad displayed as
'9 at 9@9 at ...' If you get that on the display it meant that the CPU was
running, the video circuitry was doing soemthing ,and so on.
-tony