Re: Short, short questions

[David Brant]

I thought the idea of mode2 was you could have different vectors for 
different devices connected well this throws a spanner in the works. But 
then again is there any hardware for the SAM that uses them? I think it must 
have been an old spectrum book that said this about swapping high,low bytes. 
After a little test and using old brain this is wrong.

Dave

----- Original Message ----- 
From: "Edwin Blink" <[EMAIL PROTECTED]>
To: <sam-users@nvg.ntnu.no>
Sent: Wednesday, May 21, 2008 5:34 AM
Subject: Re: Short, short questions


All 8 bits are used for LSB of the vector. The part where bit 0 always is 
zero is when one of the Z80's IO chips is connected (PIO,SIO,CTC etc) is 
connected.

Edwin

----- Original Message ----- 
From: "David Brant" <[EMAIL PROTECTED]>
To: <sam-users@nvg.ntnu.no>
Sent: Wednesday, May 21, 2008 1:02 AM
Subject: Re: Short, short questions


I've just been looking at my books. Although I can't find the bit that 
said about swapping to high,low but I'm sure that I did read it 
somewhere. It does say that the device only gives the bits 1-7 and bit 0 
is always 0 giving 128 possible addresses.

Dave

----- Original Message ----- 
From: "David Brant" <[EMAIL PROTECTED]>
To: <sam-users@nvg.ntnu.no>
Sent: Tuesday, May 20, 2008 10:49 PM
Subject: Re: Short, short questions


This was based on info from a book called z-80 Workshop manual by E.A 
Parr. The I register gives the high part of the table and the hardware 
gives the low part to the table then takes that word for the service 
routine. So if you start from one byte before the table and use the same 
address for all entries and over run it by one it will work. My demo of 
a full scrolling football pitch used this system, which I believe you 
saw many years a go.

Dave

----- Original Message ----- 
From: "Andrew Collier" <[EMAIL PROTECTED]>
To: <sam-users@nvg.ntnu.no>
Sent: Tuesday, May 20, 2008 9:50 PM
Subject: Re: Short, short questions


Hi,

I'm sceptical about this claim. I've never heard anybody say that the 
vector formed is big-endian - it's just you don't know the byte offset 
from which the interrupt vector will be fetched. (As Edwin says, it is 
usually 255 - which is odd so your 1-aligned table will usually work - 
but I don't know that Sam's hardware guarantees this).

So the high byte comes from I, the low byte from the data bus; this 
forms a 16 bit address which will be incremented once (which is why 
the table needs 257 bytes, not 256). You could, at least in theory, 
read the vector address from even or odd overlapping entries, which is 
why the usual strategy is to pick a vector address whose low and high 
bytes are the same.

The last IM2 interrupt routine I wrote looked something like this:

ds ALIGN 256
IM2TABLE: equ $
IM2BYTE: equ im2table/256

IM2TARGETBYTE:  equ IM2BYTE+1
for 257, DB IM2TARGETBYTE

IM2TARGET: equ 257*IM2TARGETBYTE
ds IM2TARGET-$

                EX   AF,AF'
...

Andrew


On 20 May 2008, at 21:16, David Brant wrote:

Mode 2 uses a table with 128 word address but as byte high,byte low 
not the normal low, high bytes

So if you set your org/dump address to &??FF (i.e. &??00-1)

and then do

          DEFW    mode2.i,mode2.i

so you have 129 words.

mode2.i:
                      di
                      push    af
                      in    a,(status.int)
                         .....
                         .....
                      ei
                      ret



----- Original Message ----- From: "Andrew Collier" 
<[EMAIL PROTECTED]
>
To: <sam-users@nvg.ntnu.no>
Sent: Tuesday, May 20, 2008 3:22 PM
Subject: Re: Short, short questions




The usual strategies are to use mode 1, or to use mode 2 with a 257- 
byte table all
containing the same byte.



--
 ---       Andrew Collier         ----
  ---- http://www.intensity.org.uk/ ---
                                      --