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Instruction Set of the I/O Processor
Read/Write Instructions
6-14
SYM53C825A/825AE Data Manual
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
SYM53C825A.
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.
Move to/from
SFBR Cycles
All operations are read-modify-writes. However,
two registers are involved, one of which is always
the SFBR. T he possible functions of this instruc-
tion are:
I
Write one byte (value contained within the
SCRIPT S instruction) into any chip register.
I
Move to/from the SFBR from/to any other
register.
I
Alter the value of a register with AND/OR/
ADD/X OR/SHIFT LEFT /SHIFT RIGHT
operators.
I
After moving values to the SFBR, the compare
and jump, call, or similar instructions may be
used to check the value.
I
A Move-to-SFBR followed by a Move-from-
SFBR can be used to perform a register to
register move.