Clear Instruction

When the SACK/or SATN/ bits are cleared, the corresponding bits are cleared in the SCSI Output Con- trol Latch (SOCL) register. When the target bit is cleared, the corresponding bit in the SCSI Control Zero (SCNTL0) register is cleared. When the Carry bit is cleared, the corresponding bit in the ALU is cleared.

RA

Relative Addressing Mode

26

 

When this bit is set, the 24-bit signed value in the DMA

 

Next Address (DNAD) register is used as a relative

 

 

displacement from the current DMA SCRIPTS Pointer

 

 

(DSP) address. Use this bit only in conjunction with the

 

Select, Reselect, Wait Select, and Wait Reselect

 

 

instructions. The Select and Reselect instructions can

 

 

contain an absolute alternate jump address or a relative

 

transfer address.

 

TITable Indirect Mode25

When this bit is set, the 24-bit signed value in the DMA Byte Counter (DBC) register is added to the value in the Data Structure Address (DSA) register, and used as an offset relative to the value in the Data Structure Address (DSA) register. The SCSI Control Three (SCNTL3) value, SCSI ID, synchronous offset and synchronous period are loaded from this address. Prior to the start of an I/O, load the Data Structure Address (DSA) with the base address of the I/O data structure. Any address on a Dword boundary is allowed. After a Table Indirect opcode is fetched, the Data Structure Address (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.

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 Dword boundary and may cross system segment boundaries. There are two restrictions on the placement of data in system memory:

The I/O data structure must lie within the 8 Mbytes above or below the base address.

I/O Instruction

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LSI 53C810A technical manual Relative Addressing Mode, TITable Indirect Mode25