LSI LSI53C1000, Technical Manual

Page: 257 / 384

Read/Write Instructions 5-23
5.4 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
SCSI First Byte
Received (SFBR) register and stores the result back to the specified
register or to the SFBR. If the COM bit ( DMA Control (DCNTL) , bit 0) is
cleared, Read/Write instructions cannot be used.
5.4.1 First Dword
This section describes the first Dword of the Read/Write Instruction
register.
Figure 5.7 Read/Write Instruction - First Dword
IT[1:0] Instruction Type - Read/Write Instruction [31:30]
The Read/Write instruction uses operator bits [26:24] in
conjunction with the opcode bits to determine which
instruction is currently selected.
OPC[2:0] Opcode [29:27]
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]
The Operator bits are used in conjunction with the
opcode bits to determine which instruction is currently
selected. Refer to
Table 5.1 for field definitions.
D8 Use data8/SFBR 23
When this bit is set, SCSI First Byte Received (SFBR) is
used, instead of the data8 value, during a Read-Modify-
Write instruction (see
Table 5.1 on page 5-25 ). This
allows the user to add two register values.
31 30 29 27 26 24 23 22 16 15 8 7 6 0
DCMD Register DBC Register
IT[1:0] OPC[2:0] O[2:0] D8 Register Address [6:0] Immediate Data 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 0 0 0 0 0 0 0 0