SBS Technologies 466-1, Hardware Manual

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Model 466-1 Adapter
70 Common Procedure And Programming Sequences
8.8 Modifying The Control Registers
The software must write "1"s and "0"s to specific bits in the control registers to
control the adapter. The software must keep track of the states of the other bits
in the control registers to not inadvertently write undesired values to the other
bits. Two methods can be used:
Read-Mask-Write. This method involves reading the register, masking
a "1" or "0" into the desired bit position, then writing the result (as a byte)
back to the register. Care must be taken with this method because reading
some I/O register locations returns different results than what was written.
For example, when placing the adapter into Page Mode, it would be an
error to read the Remote Status Register (I/O LO + 9), and OR the results
with 0x09, then write the result to the Remote Command Register 1 (I/O
LO + 9). This would inadvertently set the
LOCK Bus bit because reading
I/O LO + 9 returns a value for the LOCK Bus bit that is the inversion of
what was written.
Keep an image of the control register in system RAM. When the software
needs to change the value of a specific bit, it can perform the necessary
masking on the image. The image can then be written to the appropriate
control register. This method may seem slightly more cumbersome, but it
tends to result in more robust, reliable code.