
SYM53C876/876E Data Manual
4-49
Registers
SCSI Registers
Bit 5
SIOM (Source I/O-Memory Enable)
This bit is defined as an I/O Memory Enable
bit for the source address of a Memory Move
or Block Move Command. If this bit is set,
then the source address is in I/O space; and if
reset, then the source address is in memory
space.
This function is useful for register-to-memory
operations using the Memory Move instruc-
tion when a SYM53C876 SCSI function is
I/O mapped. Bits 4 and 5 of the CTEST2 reg-
ister determine the configuration status of the
SYM53C876 SCSI function SCSI function.
Bit 4
DIOM (Destination I/O-Memory
Enable)
This bit is defined as an I/O Memory Enable
bit for the destination address of a Memory
Move or Block Move Command. If this bit is
set, then the destination address is in I/O
space; and if reset, then the destination
address is in memory space.
This function is useful for memory–to–
register operations using the Memory Move
instruction when a SYM53C876 SCSI func-
tion is I/O mapped. Bits 4 and 5 of the
CTEST2 register determine the configuration
status of the SYM53C876 SCSI function.
Bit 3
ERL (Enable Read Line)
This bit enables a PCI Read Line command.
If this bit is set and the chip is about to exe-
cute a read cycle other than an op code fetch,
then the command is 1110.
Bit 2
ERMP (Enable Read Multiple)
If this bit is set and cache mode is enabled, a
Read Multiple command is used on all read
cycles when it is legal.
Bit 1
BOF (Burst Op Code Fetch Enable)
Setting this bit causes the SYM53C876 SCSI
function to fetch instructions in burst mode.
Specifically, the chip bursts in the first two
longwords of all instructions using a single bus
ownership. If the instruction is a memory-to-
memory move type, the third longword is
accessed in a subsequent bus ownership. If the
instruction is an indirect type, the additional
longword is accessed in a subsequent bus
ownership. If the instruction is a table indirect
block move type, the chip accesses the
remaining two longwords in a subsequent bus
ownership, thereby fetching the four long-
words required in two bursts of two longwords
each. If prefetch is enabled, this bit has no
effect. This bit also has no effect on fetches
out of SCRIPT RAM.
Bit 0
MAN (Manual Start Mode)
Setting this bit prevents the SYM53C876
SCSI function from automatically fetching
and executing SCSI SCRIPTS when the DSP
register is written. When this bit is set, the
Start DMA bit in the DCNTL register must
be set to begin SCRIPTS execution. Clearing
this bit causes the SYM53C876 SCSI func-
tion to automatically begin fetching and exe-
cuting SCSI SCRIPTS when the DSP register
is written. This bit normally is not used for
SCSI SCRIPTS operations.