OK I’m using ASM51 cross assembler from metalink and writing code for a control problem.

For my task I must use I2C to send control bytes to two different addresses. I have no problem with doing this but where it gets tricky is that I have to break up these two bytes into 2 bit sections to control individual dual input multiplexors.
So for each of the 8 multiplexors I have 4 outputs each of which can be selected to switch on one at a time using my 2 bit input.
My first attempt at this resulted in me being only able to control two multiplexors at a time, one for address 1 and the other in address 2.
The problem was that when I built my control byte I used the 2 bit mux input and then shifted it to the mux position where I wanted to make my change so it looked like -> 00000011 and then shifted -> 00110000
So this way if my previous message looked like -> 00001100, I lose my original setting.

What I really need is to be able to update my message with the two new bits that are needed.

Is it possible to set up the code so I can change only 2 bits of a byte at a time

Here is what I was thinking of doing

********************************************************
MESSAGE	DATA	0x50		; byte containing previous message
MUX_BIT_NO	DATA	0x51		; byte containing the position of the first bit to be changed

BIT_1		BIT	0x08		; first of 2 bit change
BIT_2		BIT	BIT_1+1	; second of 2 bit change
-----------------------------------------------------------------------------------------
MOV	A, MESSAGE			; reload the old message into ACC
MOV	A.MUX_BIT_NO, BIT_1		; change first bit
MOV	A.MUX_BIT_NO+1, BIT_2	; change second bit
MOV MESSAGE, A			; save new message
RET
----------------------------------------------------------------------------------------
END
*******************************************************

Let me know if you think that this might work, or if there is an easier way to solve the problem