If relative or table relative addressing is used, this value is a 24-bit signed offset relative to the current DMA SCRIPTS Pointer (DSP) 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 SCSI First Byte 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.

5.5.1 First Dword

31 30 29

27 26

24 23 22

16 15

 

8

7

6

0

DMA Command (DCMD)

 

 

DMA Byte Counter (DBC) 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

 

 

 

 

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 Table 5.1 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 Table 5.1). This allows the user to add two register values.

5-22

SCSI SCRIPTS Instruction Set

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LSI 53C875A Read/Write Instructions, IT10 Instruction Type Read/Write Instruction 3130, O20 Operator 2624, Use data8/SFBR