Bit Manipulation Instructions (Setb And Clrb); Read-Modify-Write Operation - Fujitsu F2MC-8L F202RA Hardware Manual

B.3

Bit Manipulation Instructions (SETB and CLRB)

Some registers of peripheral functions have bits that perform a read operation different
from ordinary read for a bit manipulation instruction.
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Read-modify-write Operation

The bit manipulation instructions can set "1" (SETB) to the specified bit in a register or RAM or clear it to
"0" (CLRB). Because the CPU handles the data in 8 bits, however, it actually reads the 8-bit data, modifies
the specified bit, and then writes it back to the original address. This series of operations is called read-
modify-write operation.
Table B.3-1 shows the bus operation at bit manipulation instructions.
Table B.3-1 Bus Operation at Bit Manipulation Instructions
CODE MNEMONIC
A0 to A7 CLRB dir:b
A8 to AF SETB dir:b
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Read Destination at Execution of a Bit Manipulation Instruction
For some I/O ports and interrupt request flag bits, the read destination for read-modify-write is different
from that for ordinary read.
● I/O port (at bit manipulation)
For some I/O ports, the value of the I/O pin is read at ordinary read; meanwhile, the value of output latch is
read at bit manipulation. This is to prevent the other bits of the output latch from being accidentally
changed regardless of the I/O direction and pin state.
● Interrupt request flag bit (at bit manipulation)
The interrupt request flag bits work as flag bits for confirming an interrupt request at ordinary read;
meanwhile, "1" is always read at bit manipulation. This is to prevent the interrupt request flag bits from
being written as "0", and accidentally clearing the flags at bit manipulation for another bit.
~ Cycle Address bus
4 1 N+1
2 dir address
3 dir address
4 N+2
Data bus RD
Dir 0
Data 0
Data 1
Next operation 0
WR RMW
1 0
1 1
0 0
1 0
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