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Instruction Set of the I/O Processor
Read/Write Instructions
SYM53C875/875E Data Manual
6-13
Read/Write Instructions
T he Read/Write instruction supports addition,
subtraction, and comparison of two separate values
within the chip. It performs the desired operation
on the specified register and the SFBR register,
then stores the result back to the specified register
or the SFBR.
First Dword
Bits 31-30 Instruction Type - Read/Write
Instruction
T he Read/Write instruction uses operator bits
26 through 24 in conjunction with the op code
bits to determine which instruction is currently
selected.
Bits 29-27 Op Code
T he combinations of these bits determine if the
instruction is a Read/Write or an I/O instruc-
tion. Op codes 000 through 100 are considered
I/O instructions.
Bits 26-24 Operator
T hese bits are used in conjunction with the op
code bits to determine which instruction is
currently selected. Refer to table 6-1 for field
definitions.
Bit 23
Use data8/SFBR
When this bit is set, SFBR will be used instead
of the data8 value during a Read-Modify-Write
instruction (see Table 6-1). T his allows the
user to add two register values.
Bits 22-16 Register Address - A(6-0)
Register values may be changed from
SCRIPT S in read-modify-write cycles or move
to/from SFBR cycles. A(6-0) select an 8-bit
source/destination register within the
SYM53C875.
Bits 15-8 Immediate Data
T his 8-bit value is used as a second operand in
logical and arithmetic functions.
Bits 7-0 Reserved
Second Dword
Bits 31-0Destination Address
T his field contains the 32-bit destination
address where the data is to be moved.
Read-Modify-Write
Cycles
During these cycles the register is read, the
selected operation is performed, and the result is
written back to the source register.
T he Add operation can be used to increment or
decrement register values (or memory values if
used in conjunction with a Memory-to-Register
Move operation) for use as loop counters.
Subtraction is not available when SFBR is used
instead of data8 in the instruction syntax. To sub-
tract one value from another when using SFBR,
first X OR the value to subtract (subtrahend) with
0xFF, and add 1 to the resulting value. T his creates
the 2’s compliment of the subtrahend. T he two val-
ues can then be added to obtain the difference.