2 second dword, 3 read-modify-write cycles, Second dword – LSI 53C875A User Manual
Page 223: Read-modify-write cycles
Read/Write Instructions
5-23
A[6:0]
Register Address - A[6:0]
[22:16]
It is possible to change register values from SCRIPTS in
read-modify-write cycles or move to/from SFBR cycles.
A[6:0] selects an 8-bit source/destination register within
the LSI53C875A.
ImmD
Immediate Data
[15:8]
This 8-bit value is used as a second operand in logical
and arithmetic functions.
A7
Upper Register Address Line [A7]
7
This bit is used to access registers 0x80–0xFF.
R
Reserved
[6:0]
5.5.2 Second Dword
Destination Address
[31:0]
This field contains the 32-bit destination address where
the data is to move.
5.5.3 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.
The Add operation is 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 subtract one value from another when using SFBR,
first XOR the value to subtract (subtrahend) with 0xFF, and add 1 to the
resulting value. This creates the 2’s complement of the subtrahend. The
two values are then added to obtain the difference.
31
0
DMA SCRIPTS Pointer Save (DSPS)
Register
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
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x
x
x
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x
x
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x