Same MPU, diff proms 512 and 1024

[Wayne Blair]

>From: "TBK" <terryk at foothill.net>
>Date: Wed, 13 May 1998 21:47:49 -0700
>Subject: Re: Same MPU, different proms

>All processors have what is called the Restart or Reset Vector. When the
>processor starts up, it loads the program counter with address of the start
>of the code. For the Motorola's, this start up address is FFFE and FFFF
>(double byte). Located here is the address of the start of the program code.
>This address is loaded into the program counter and off it goes.

<Tip 1 for disassembly>

>- A0 00  (FFFE and FFFF). This means the start of code is address A000. The
>start of the code is almost alway located after the data (although having
>some data scattered in the middle of the code is not that unusual).

<Tip 2 for disassembly>

>Since the processor damn well expects the Restart Vector at FFFF, the
>program must but pushed up to this address. The processor has to start
>somewhere. And this is the somewhere.

>128K rom is located from C000 to FFFF and a 
>256k is from 8000-FFFF

512K is from 0 - FFFF
NOW how are the 1024K(32 pins) paged ??? 
I'm pretty sure the same CPU is used but  how ???

>But when you look at a rom in a hex editor, it shows it as "physical"
>addresses. The first byte in the rom is at address 0000 and the last byte is
>3FFF for a 128K rom and 7FFF for a 256k rom. The hardware is wiring to make
>the rom "appear" as being at address C000-FFFF for the 128k and 8000-FFFF
>for the 256k.

>How is another subject for another time.

But for now think about the following:
128,256,512 are all 28 pin chips, check their pin outs and think about the
address 
lines used and not used and it may become clear 
how each chip can be used in the same "hole" 

>If you look at group of two bytes backwards from FFFE and FFFF (say, FFFC
>and FFFD), these are the addresses where the process should go if a hardware
>interupt occurs, or if a timer overflows, or whenever some event causes the
>processor to jump to some other code. I think the Motorola HC has some 10
>vectors ("where do I go it this happens").

<Tip 3 for disassembly>

wayne