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Operating Registers
5-34
SYM53C810A Data Manual
Register 3B (BB)
DMA Control (DCNT L)
Read/Write
Bit 7
CLSE (Cache Line Size E nable)
Setting this bit enables the SYM53C810A to
sense and react to cache line boundaries set up
by the DMODE or PCI Cache Line Size regis-
ter, whichever contains the smaller value.
Clearing this bit disables the cache line size
logic and the SYM53C810A monitors the
cache line size via the DMODE register.
Bit 6
PFF (Pre-Fetch Flush)
Setting this bit will cause the pre-fetch unit to
flush its contents. T he bit will reset after the
flush is complete.
Bit 5
PFE N (Pre-fetch E nable)
Setting this bit enables the pre-fetch unit if the
burst size is equal to or greater than four. For
more information on SCRIPT S instruction
prefetching, see Chapter 2.
Bit 4
SSM (Single-step mode)
Setting this bit causes the SYM53C810A to
stop after executing each SCRIPT S instruc-
tion, and generate a single step interrupt.
When this bit is clear the SYM53C810A will
not stop after each instruction; instead it con-
tinues fetching and executing instructions until
an interrupt condition occurs. T his bit should
be clear for normal SCSI SCRIPT S operation.
To restart the SYM53C810A after it generates
a SCRIPT S Step interrupt, read the ISTAT
and DSTAT registers to recognize and clear
the interrupt; then set the START DMA bit in
this register.
Bit 3
IRQM (IRQ Mode)
When set, this bit enables a totem pole driver
for the IRQ pin. When reset, this bit enables an
open drain driver for the IRQ pin with a inter-
nal weak pull-up. T his bit is reset at power up.
Bit 2
ST D (Start DMA operation)
T he SYM53C810A fetches a SCSI SCRIPT S
instruction from the address contained in the
DSP register when this bit is set. T his bit is
required if the SYM53C810A is in one of the
following modes:
1. Manual start mode – Bit 0 in the DMODE
register is set
2. Single-step mode – Bit 4 in the DCNT L
register is set
When the SYM53C810A is executing
SCRIPT S in manual start mode, the Start
DMA bit needs to be set to start instruction
fetches. T his bit will remain set until an inter-
rupt occurs. When the SYM53C810A is in sin-
gle-step mode, the Start DMA bit needs to be
set to restart execution of SCRIPT S after a sin-
gle-step interrupt.
Bit 1
IRQD (IRQ Disable)
Setting this bit tristates the IRQ pin; clearing
the bit enables normal operation. When bit 1 in
this register is set, the IRQ/ pin will not be
asserted when an interrupt condition occurs.
T he interrupt is not lost or ignored, but merely
masked at the pin. Clearing this bit when an
interrupt is pending will immediately cause the
IRQ/ pin to assert. As with any register other
than ISTAT, this register cannot be accessed
except by a SCRIPT S instruction during
SCRIPT S execution.
CLSE
7
PFF
6
PFEN
5
SSM
4
IRQM
3
STD
2
IRQD
1
COM
0
Default>>>
0
0
0
0
0
0
0
0