Direct Addressing

The byte count and absolute address are:

Command

Byte Count

 

 

Address of Data

Indirect Addressing

Use the fetched byte count, but fetch the data address from the address in the instruction.

Command

Byte Count

 

 

Address of Pointer to Data

Once the data pointer address is loaded, it is executed as when the chip operates in the direct mode. This indirect feature allows a table of data buffer addresses to be specified. Using the SCSI SCRIPTS assembler, the table offset is placed in the script at compile time. Then at the actual data transfer time, the offsets are added to the base address of the data address table by the external processor. The logical I/O driver builds a structure of addresses for an I/O rather than treating each address individually. This feature makes it possible to locate SCSI SCRIPTS in a PROM.

Note:

Do not use indirect and table indirect addressing

 

 

simultaneously; use only one addressing method at a time.

TIA

Table Indirect

28

 

32-Bit Addressing

 

When this bit is set, the 24-bit signed value in the start address of the move is treated as a relative displacement from the value in the Data Structure Address (DSA) register. Both the transfer count and the source/ destination address are fetched from this location.

Use the signed integer offset in bits [23:0] of the second four bytes of the instruction, added to the value in the Data Structure Address (DSA) register, to fetch first the byte count and then the data address. The signed value is combined with the data structure base address to generate the physical address used to fetch values from

Block Move Instruction

5-7

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LSI 53C875A technical manual Direct Addressing, Tia, Table Indirect Bit Addressing