the LSI53C875A to make more efficient use of the system PCI bus, thus improving overall system performance. The unit will flush whenever the PFF bit is set, as well as on all transfer control instructions when the transfer conditions are met, on every write to the DMA SCRIPTS Pointer (DSP), on every regular MMOV instruction, and when any interrupt is generated. The unit automatically determines the maximum burst size that it is capable of performing based on the burst length as determined by the values in the DMA Mode (DMODE) register. If the burst threshold is set to 8 Dwords the prefetch unit will fetch instructions in two bursts of

4 Dwords. If the burst threshold is set to 16 Dwords or greater the prefetch unit will fetch instructions in one burst of 8 Dwords. Burst thresholds of less than 8 Dwords will cause the prefetch unit to be disabled. PCI Cache commands (Read Line and Read Multiple) will be issued appropriately if PCI caching is enabled. Prefetching from SCRIPTS RAM is not supported and is unnecessary due to the speed of the fetches. When fetching from SCRIPTS RAM the setting of this bit will have no effect on the fetch mechanism from SCRIPTS RAM.

SSM

Single Step Mode

4

 

Setting this bit causes the LSI53C875A to stop after

 

 

executing each SCRIPTS instruction, and generate a

 

 

single step interrupt. When this bit is cleared the

 

 

LSI53C875A does not stop after each instruction. It

 

 

continues fetching and executing instructions until an

 

 

interrupt condition occurs. For normal SCSI SCRIPTS

 

 

operation, keep this bit clear. To restart the LSI53C875A

 

after it generates a SCRIPTS Step interrupt, read the

 

 

Interrupt Status Zero (ISTAT0), Interrupt Status One

 

 

(ISTAT1) and DMA Status (DSTAT) registers to recognize

 

and clear the interrupt. Then set the START DMA bit in

 

this register.

 

IRQMIRQ Mode3

When set, this bit enables a totem pole driver for the IRQ/ pin. When cleared, this bit enables an open drain driver for the IRQ/ pin with an internal weak pull-up. The bit should remain cleared to retain full PCI compliance.

SCSI Registers

4-71

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Image 163
LSI 53C875A technical manual Ssm, Single Step Mode, Irqmirq Mode3