the data structure. Sign extended values of all ones for negative values are allowed, but bits [31:24] are ignored.

Command

Not Used

 

 

Don’t Care

Table Offset

 

 

Note: Do not use indirect and table indirect addressing simultaneously; use only one addressing method at a time.

Prior to the start of an I/O, the Data Structure Address (DSA) register should be loaded with the base address of the I/O data structure. The address may be any address on a longword boundary.

After a Table Indirect opcode is fetched, the DSA is added to the 24-bit signed offset value from the opcode to generate the address of the required data; both positive and negative offsets are allowed. A subsequent fetch from that address brings the data values into the chip.

For a MOVE instruction, the 24-bit byte count is fetched from system memory. Then the 32-bit physical address is brought into the LSI53C875A. Execution of the move begins at this point.

SCRIPTS can directly execute operating system I/O data structures, saving time at the beginning of an I/O operation. The I/O data structure can begin on any longword boundary and may cross system segment boundaries.

There are two restrictions on the placement of pointer data in system memory:

The eight bytes of data in the MOVE instruction must be contiguous, as shown below, and

Indirect data fetches are not available during execution of a Memory-to-Memory DMA operation.

00

Byte Count

 

 

Physical Data Address

5-8

SCSI SCRIPTS Instruction Set

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LSI 53C875A technical manual Command Not Used Don’t Care