STDStart DMA Operation2

The LSI53C875A fetches a SCSI SCRIPTS instruction from the address contained in the DMA SCRIPTS Pointer (DSP) register when this bit is set. This bit is required if the LSI53C875A is in one of the following modes:

Manual start mode – Bit 0 in the DMA Mode (DMODE) register is set

Single step mode – Bit 4 in the DMA Control (DCNTL) register is set

When the LSI53C875A is executing SCRIPTS in manual start mode, the Start DMA bit must be set to start instruction fetches, but need not be set again until an interrupt occurs. When the LSI53C875A is in single step mode, set the Start DMA bit to restart execution of SCRIPTS after a single step interrupt.

IRQD

IRQ Disable

1

 

Setting this bit disables the IRQ pin. Clearing the bit

 

 

enables normal operation. As with any other register

 

 

other than Interrupt Status Zero (ISTAT0) and Interrupt

 

 

Status One (ISTAT1), this register cannot be accessed

 

 

except by a SCRIPTS instruction during SCRIPTS

 

 

execution. For more information on the use of this bit in

 

interrupt handling, see Chapter 2, “Functional

 

 

Description.”

 

COMLSI53C700 Compatibility0

When the COM bit is cleared, the LSI53C875A behaves in a manner compatible with the LSI53C700; selection/reselection IDs are stored in both the SCSI Selector ID (SSID) and SCSI First Byte Received (SFBR) registers. This bit is not affected by a software reset.

If the COM bit is cleared, do not access this register using SCRIPTS operation as nondeterminate operations may occur. (This includes SCRIPTS Read/Write operations and conditional transfer control instructions that initialize the SFBR register.)

When the COM bit is set, the ID is stored only in the SSID register, protecting the SFBR from being overwritten if a selection/reselection occurs during a DMA register-to-register operation.

4-72Registers

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LSI 53C875A technical manual STDStart DMA Operation2, Irqd, IRQ Disable, COMLSI53C700 Compatibility0