ORG help for a newbie!

MCassidy

Hello all,
I'm trying to get the hang of this ASM programming stuff (on SX 28), but I'm having some trouble with a few key concepts, especially memory mapping.· I understand that the lower half of each bank is always the same set of registers ($00-$0F), and that $03, $05, etc. represent different banks of data registers (8 total)·and that there are 4 program data pages, but I get confused with using the ORG instruction.·

Can someone point me towards some documentation that clearly explains it?

Thanks.

-MichaelC.

Bean

ORG just sets the address of the next instruction. It has NOTHING to do with the RAM banks.

Bean.

Guenther Daubach

Michael,

here is a code snippet copied from one of my programs:

; Global variables
;
org 8
Global_ByteCount ds 1
Global_Index ds 1
; more (max. 8 variables)

org $30 ; Serial variable area
Serial = $
Serial_TxHighPC ds 1
Serial_TxLowPC ds 1
; more (max. 16 variables)...

org $50 ; FIFO buffer
Buffer ds 16 ; 16-byte array, indirectly addressed

org $70 ; Miscellaneous stuff
Misc = $
Misc_FIFOHead ds 1
Misc_FIFOTail ds 1
; more (max. 16 variables)...

org $90
Free1 = $

org $b0
Free2 = $

org $d0
Free3 = $

org $f0
Free4 = $

Here, the org directives tell the assembler where (i.e. in which bank) it shall set aside memory for the variables. I·always also define a symbolic name for each bank, e.g. Serial = $. Later, when you want to access a variable, be careful that the most recently executed bank instruction has selected the bank, where the variable is located. If necessary, insert an instruction like "bank Serial" before accessing TXHighPC in my sample code.

In the meantime, for variables, I strictly use the naming convention <Bank>_<VarName>, i.e. the first part of each name consists of the bank name. This makes the code easier to read because you always know, where a variable is located, and helps to check if the right bank is selected.

BTW: The ORG directive is also used to specify the starting points of program code, like in

org 0

; The interrupt service routine must always start here (if any)
;
ISR
; some code
mov w, #IntPeriod
retiw

org $100

; The Main program
;
Main
; instructions following

The assembler is clever enough to find out when an ORG is used to specify a location in data memory, or in program memory.

Each memory bank has a size of 32 bytes where the lower half, i.e. the first 16 bytes are always mapped to bank 0, no matter which bank is actually selected. Within the first 16 lower bytes of bank 0, only the higher eight locations are free to be used as variable space because the lower eight bytes are used as special registers, e.g. the PC, the FSR, the ports, etc.

Therefore, there is the ORG 8 in the above example. As these upper eight bytes in bank 0 can always be accessed, no matter which bank is actually selected, they are usualy called "Global Variables".

More confused than before ?

Greetings,

Günther

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Post Edited (Guenther Daubach) : 8/27/2004 7:03:17 PM GMT

Catware

Thanks a lot Guenther; your explanation was really useful to me! It also helped simplify my code a lot.··