SETB problem

Sens-a-Dat

Gents,

I am trying to use a variable to help me set select bits within another variable. The bits set will vary, and thus the desire to use a variable. What I think should work does not. So, I know I must not be understanding something. Can someone help me on this?

Here is an example code to illustrate how I am trying to manipulate bits in TestByte by using the variable BitLocation as a bit positioner. The counter will go from 7 to 0, so that all bits will be set for this example.

I am using the SXSim to monitor the process. The BitLocation does get smaller, but the use of it in the SETB command is not reflected.

DEVICE SX28, TURBO, STACKX, OSC4MHZ
IRC_CAL IRC_4MHZ
FREQ 4_000_000


RESET   Start

;************************ Program Variables *****************************

org     $08                          ;First register address in main memory
                                     ;bank
TestByte        ds      1            ;Temporary storage
BitLocation     ds      1            ;Bit location within TestByte


org $100
Start
break

mov BitLocation,#$07           ;location of bit to be set in TestByte
mov TestByte,#$00              ;Initial value = 0

Main
    SETB TestByte.BitLocation
    DEC BitLocation
JMP Main

JonnyMac

I don't think you can ue a variable to do that -- you need to convert the bit position to a mask and then OR the two values together.

Bean

Jon is correct. The bit position must be a constant when using the SETB instruction.

Here is how I would do it using a mask value and "OR":

DEVICE SX28, TURBO, STACKX, OPTIONX, OSC4MHZ
IRC_CAL IRC_4MHZ
FREQ 4_000_000


RESET   Start

;************************ Program Variables *****************************

ORG     $08                          ;First register address in main memory
                                     ;bank
testByte        ds      1            ;Temporary storage
bitLocation     ds      1            ;Bit location within TestByte


ORG $100
Start:
BREAK

MOV bitLocation,#$80           ; mask value of bit to be set in TestByte
MOV testByte,#$00              ; Initial value = 0

CLC                            ; clear carry so we shift a zero on the first shift
 
Main:

  OR TestByte,BitLocation      ; Set the bit
  RR BitLocation               ; Adjust so next bit is a "1"
JNC Main                       ; Quit when the "1" bit gets shifted out to the carry bit
 
JMP $                          ; Stop here

Bean

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Post Edited (Bean (Hitt Consulting)) : 2/27/2008 1:45:38 PM GMT

Sens-a-Dat

Thank you, Bean. Your code helped me in better understanding how to look at and consider things.