require certain unique timings or bus sequences to operate properly. Another feature allowed at the low level is loopback testing. In loopback mode, the SCSI core can be directed to talk to the DMA core to test internal data paths all the way out to the chip’s pins.

5.2 High Level SCSI SCRIPTS Mode

To operate in the SCSI SCRIPTS mode, the LSI53C875A 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. 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 LSI53C875A 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 LSI53C875A 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

LSI53C875A, as shown in Table 5.1:

5-2

SCSI SCRIPTS Instruction Set

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LSI 53C875A technical manual High Level Scsi Scripts Mode