Read/Write Instructions; First Dword - LSI LSI53C875A Technical Manual

5.5 Read/Write Instructions

5.5.1 First Dword

31 30 29 27 26
DMA Command (DCMD)
Register
IT[1:0] OPC[2:0] O[2:0]
0 1 x x x x x
5-22
If relative or table relative addressing is used, this value
is a 24-bit signed offset relative to the current
SCRIPTS Pointer (DSP)
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
Received (SFBR) 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.
24 23 22
D8 A[6:0]
x x x x x x x
IT[1:0] Instruction Type - Read/Write Instruction
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
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
These bits are used in conjunction with the opcode bits
to determine which instruction is currently selected. Refer
to Table 5.1
D8 Use data8/SFBR
When this bit is set, SFBR is used instead of the data8
value during a Read-Modify-Write instruction (see
Table
values.
SCSI SCRIPTS Instruction Set
register value.
16 15
DMA Byte Counter (DBC)
ImmD
x x x x x x x
for field definitions.
5.1). This allows the user to add two register
SCSI First Byte
8 7 6
Register
A7 Reserved - Must be 0
x x x x 0 0 0
DMA
0
0 0 0 0
[31:30]
[29:27]
[26:24]
23