Re: Short, short questions

[Andrew Collier]

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/ ---
                                      --