Using an Array Element as an Array Index

JonnyMac · 2009-02-15 19:02

Given this kind of declaration:

tmpArray        VAR     Byte (9)
tmpIdx          VAR     tmpArray(8)

... couldn't SX/B allow tmpIdx to be used as an index into tmpArray?

tmpArray(tmpIdx) = X

... where X is evaluated and moved into __PARAM1 -- this code seems to work:

MOV   FSR, #tmpArray
ADD   FSR, tmpIdx
MOV   IND, __PARAM1
MOV   FSR, #__DEFAULT

My suggestion is that if the an array element can be used as an index into an array that it's part of -- knowing nothing about how the compiler validates and generates code I don't know if this is a reasonable request or not.

JonnyMac · 2009-02-15 19:12

Continuing the thought -- with full recognition that Assembly is not my first language -- could an element of another array be used (so long as it's aliased) if the did this:

MOV   FSR, #idx2
MOV   __PARAM4, IND
MOV   FSR, #tmpArray
ADD   FSR, __PARAM4
MOV   IND, __PARAM1
MOV   FSR, #__DEFAULT

In this case idx2 is an element of another array.

Peter Verkaik · 2009-02-15 21:12

Jonnymac,

I had simular thoughts expressed in the beta forum.
Here is what I came up with then:

Hi,
I know it is not supported to use an array variable as index into an array variable.
Example:

DEVICE SX28, OSC4MHZ
 
Glob var byte
WGlob var word
Norm var byte
WNorm var word
Arr var byte (1)
Index var byte (1)
Waste var byte (13)
WArr var byte (2) span
Index2 var byte (1)
 
WArr(Norm) = Arr(Index) + WArr(Index2)

throws error INVALID NUMBER OF PARAMETERS.

But I think I figured out a way to make it happen. In fact, the solution is equal
to the way large arrays on the sx28 are implemented.

Here is the code that implements the above formula on the sx28:

  'WArr(Norm) = Arr(Index) + WArr(Index2)
asm
  MOV FSR,#index                
  MOV FSR,IND                     
  AND W,#$F0                    
  ADD FSR,W                     
  ADD FSR,#Arr                     
  SETB FSR.4                    
  MOV __PARAM1,IND              
  BANK $00  
                    
  MOV FSR,#index2               
  MOV FSR,IND                     
  AND W,#$F0                    
  ADD FSR,W                     
  ADD FSR,#WArr                     
  SETB FSR.4                    
  ADD __PARAM1,IND              
  BANK $00 
endasm                     
  WArr(Norm) = __PARAM1

Resume:
To get the address of an array element whose index·is also an array element
(in pseudo code: fsr = @Arr(index) )

  ;SX18/20/28: fsr = Arr(Index)
  MOV FSR,#index                
  MOV FSR,IND                     
  AND W,#$F0                    
  ADD FSR,W                     
  ADD FSR,#Arr                     
  SETB FSR.4

  ;SX48/52: fsr = Arr(Index)
  MOV FSR,#index                
  MOV FSR,IND                     
  ADD FSR,#Arr

It looks to me the restriction that an array index must be a normal variable
is easy to resolve (without using any __PARAMx variable).

Jonnymac,
Your last example could be done without using __param4 by doing
mov fsr,#idx2
mov fsr,ind ;your __param4 value
add fsr,#tmpArray
mov ind,__param1

The key point is to calculate fsr without using any temporary variables.

regards peter

David Bayliss · 2009-02-15 22:23

In the case where the index of the nested array is constant it is easy as you say. In fact for the (fairly common) case where the array is contained within a single bank I believe you can do:

·mov fsr,#idx
·mov w,#arr1
·add w,ind
·mov fsr,w

it is a little more exciting which the index of the nested array is variable to arr1[noparse]/noparse]arr2[noparse][[/noparse]i = 42 but still not impossible:

arr1 equ $3a 
arr2 equ $5a 
i equ $a 
val equ $8 
org 0 
reset Start 
Start: 
; Preamble to allow 'stepping through' in SXSIM
mov i,#2 
mov fsr,#arr2+2 
mov ind,#3 
mov val,#$42 
; Attempting arr1[noparse][[/noparse]arr2[noparse][[/noparse]i]] = val - arr1[noparse][[/noparse]arr2[noparse][[/noparse]2]] - arr1[noparse][[/noparse]3] in test = $3d 
mov w,#arr2 
; requires a bank instruction here if i not 'already available'
add w,i 
mov fsr,w 
mov w,#arr1 
add w,ind 
mov fsr,w 
; Job done - line below as a 'proof'
mov ind,val

As in Peter's post if Arr1 or Arr2 span banks it is a little uglier ....

David
·

Peter Verkaik · 2009-02-15 22:54

David,

In theory you can expand to any level of indirection,
but that requires a change to the way sxb evaluates expressions.
Currently, you can have only one operator per line as in
; Attempting arr1[noparse]/noparse]arr2[noparse][[/noparse]i = val - arr1[noparse]/noparse]arr2[noparse][[/noparse]2
More complex expressions must be split over several lines.
What jonnymac proposed, and me too, is·to allow
an index at a fixed location, either a normal variable or an aliased
array·variable. Then all that needs to change is the calculation
of fsr. It requires no change on how expressions are evaluated.
If the index is a normal variable, use·the current way·of calculating fsr,
if the index is an array variable, use the proposed way·of calculating fsr.
The remaining generated code would remain the same.

regards peter
·

David Bayliss · 2009-02-15 23:36

Well - my comment was really a multi-line comment on one line. The expression was

arr1[noparse]/noparse]arr2[i = x

and of course you are right - this is something not presently handled. I understand what you are proposing and I agree with your code and offered my simpler version for the easy case.

Knowing just how high-level you want a high-level language to be is always hard....

David [/i]

Using an Array Element as an Array Index

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