Re: A86: im 2

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Re: A86: im 2


>I would like to thank Jimmy Merdell for his information about interupts.
Is this information located on a web page of some sort?  I would be 
interested
in seeing it too

>Also, what is the difference between RET($C9), RETI ($ED$4D), and RETN
>($ED$45)?  I'm confused I just thought that, ret was roughly equivalent
>to : POP PC.
This is from 'Mastering Machine Code On Your ZX 81' - another z-80 based 
machine:

RET - RET is used to return from a subroutine.  It works by popping an 
address
from the top of the stack, and then jumping to that address.

RETI - Used to end an interrupt subroutine.  Its function is the same as 
RET,
but RETI must be used instead of RET because the chip does clever things 
if
you get a second interrupt in the middle of the interrupt subroutine!  As 
soon
as an interrupt subroutine is called a DI instruction is automatically 
executed,
but there are such things as non-maskable interrupts, that is almighty 
superhigh-
powered interrupts that override even DI, these can cause confusion if 
you don't
use RETI.

RETN - Used to end a non-maskable interrupt subroutine.  Its function is 
the
same as RETI except that the Interrupt Mode (which was altered by the 
non-maskable
interrupt in the first place) is also restored to its previous value.

>And what exactly is the databus, is it a pin on the pinout
>of the Z80.
actually, it's 8 pins on the z80.  it's where all of the data gets passed
between the z80, memory, and the i/o ports (someone correct me if i am 
wrong)

>Also this is stupid but, when the z80 gets power, i.e.
>boots where does the PC point (I mean when batteries are first put it
>not when the ON button is pressed, because doesn't the calc just
>susppened itself and wait for the ON interupt when you turn it off?)?
the pc is initialized to 0000.  The code at 0000 in ROM page 0 is the 
code 
that is executed when the calc first gets power.

>
>Thank you for all your help, even though I may sould like an idiot.
>
>   -Gen_Witt
>     Gen_Witt@aloradus.com
>


Hope this helped



Dennis

Maintainer of the TI-8x Emulation Project:
http://members.aol.com/smiley0242