5 read/write instructions, 1 first dword, Read/write instructions – LSI 53C875A User Manual
Page 222: First dword, Section, 5, “read/write instructions
5-22
SCSI SCRIPTS Instruction Set
If relative or table relative addressing is used, this value
is a 24-bit signed offset relative to the current
register value.
5.5 Read/Write Instructions
The 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
register, then stores the result back to the specified
register or the SFBR. If the COM bit DMA Control (DCNTL bit 0) is
cleared, Read/Write instructions cannot be used.
5.5.1 First Dword
IT[1:0]
Instruction Type - Read/Write Instruction
[31:30]
The configuration of the IT bits, the OpCode bits and the
Operator bits define the Read/Write Instruction Type. The
configuration of all these bits determine which instruction
is currently selected.
OPC[2:0]
OpCode
[29:27]
The combinations of these bits determine if the
instruction is a Read/Write or an I/O instruction. OpCodes
0b000 through 0b100 are considered I/O instructions.
O[2:0]
Operator
[26:24]
These bits are used in conjunction with the opcode bits
to determine which instruction is currently selected. Refer
to
for field definitions.
D8
Use data8/SFBR
23
When this bit is set, SFBR is used instead of the data8
value during a Read-Modify-Write instruction (see
). This allows the user to add two register
values.
31 30 29
27 26
24 23 22
16 15
8
7
6
0
Register
Register
IT[1:0] OPC[2:0]
O[2:0]
D8
A[6:0]
ImmD
A7
Reserved - Must be 0
0
1
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
0
0
0
0
0
0
0