6.2 SCSI SCRIPTS

To operate in the SCSI SCRIPTS mode, the LSI53C810A requires only a SCRIPTS start address. The start address must be at a Dword (four byte) boundary. This aligns subsequent SCRIPTS at a Dword boundary since all SCRIPTS are 8 or 12 bytes long. All instructions are fetched from external memory. The LSI53C810A fetches and executes its own instructions by becoming a bus master on the host bus and fetching two or three 32-bit words into its registers. Instructions are fetched until an interrupt instruction is encountered, or until an unexpected event (such as a hardware error) causes an interrupt to the external processor.

Once an interrupt is generated, the LSI53C810A halts all operations until the interrupt is serviced. Then, the start address of the next SCRIPTS instruction may be written to the DMA SCRIPTS Pointer (DSP) register to restart the automatic fetching and execution of instructions.

The SCSI SCRIPTS mode of execution allows the LSI53C810A to make decisions based on the status of the SCSI bus, which offloads the microprocessor from servicing the numerous interrupts inherent in I/O operations.

Given the rich set of SCSI oriented features included in the instruction set, and the ability to re-enter the SCSI algorithm at any point, this high level interface is all that is required for both normal and exception conditions. Switching to low level mode for error recovery should never be required.

The following types of SCRIPTS instructions are implemented in the

LSI53C810A as shown in Table 6.1:

6-2

Instruction Set of the I/O Processor

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LSI 53C810A technical manual Scsi Scripts