![](http://datasheet.mmic.net.cn/390000/SYM53C875_datasheet_16836337/SYM53C875_122.png)
SCSI Operating Registers
5-36
SYM53C875/875E Data Manual
Bit 1
BOF (Burst Op Code Fetch E nable)
Setting this bit causes the SYM53C875 to
fetch instructions in burst mode. Specifically,
the chip will burst in the first two dwords of all
instructions using a single bus ownership. If
the instruction is a memory-to-memory move
type, the third dword will be accessed in a sub-
sequent bus ownership. If the instruction is an
indirect type, the additional dword will be
accessed in a subsequent bus ownership. If the
instruction is a table indirect block move type,
the chip will access the remaining two dwords
in a subsequent bus ownership, thereby fetch-
ing the four dwords required in two bursts of
two dwords each. T his bit has no effect if
SCRIPT S instruction prefetching is enabled.
Bit 0
MAN (Manual Start Mode)
Setting this bit prevents the SYM53C875 from
automatically fetching and executing SCSI
SCRIPT S when the DSP register is written.
When this bit is set, the Start DMA bit in the
DCNT L register must be set to begin
SCRIPT S execution. Clearing this bit causes
the SYM53C875 to automatically begin fetch-
ing and executing SCSI SCRIPT S when the
DSP register is written. T his bit normally is not
used for SCSI SCRIPT S operations.
Register 39 (B9)
DMA Interrupt Enable (DIEN)
Read/Write
T his register contains the interrupt mask bits corre-
sponding to the interrupting conditions described
in the DST AT register. An interrupt is masked by
clearing the appropriate mask bit. Masking an in-
terrupt prevents IRQ/ from being asserted for the
corresponding interrupt, but the status bit will still
be set in the DST AT register. Masking an interrupt
will not prevent the IST AT DIP from being set. All
DMA interrupts are considered fatal, therefore
SCRIPT S will stop running when this condition
occurs, whether or not the interrupt is masked. Set-
ting a mask bit enables the assertion of IRQ/ for the
corresponding interrupt. (A masked non-fatal in-
terrupt will not prevent un-masked or fatal inter-
rupts from getting through; interrupt stacking
begins when either the IST AT SIP or DIP bit is
set.)
T he SYM53C875 IRQ/ output is latched; once as-
serted, it will remain asserted until the interrupt is
cleared by reading the appropriate status register.
Masking an interrupt after the IRQ/ output is as-
serted will not cause IRQ/ to be deasserted.
For more information on interrupts, see Chapter 2,
“Functional Description.”
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Reserved
MDPE (Master Data Parity E rror)
BF (Bus Fault)
ABRT (Aborted)
SSI (Single -step Interrupt)
SIR (SCRIPT S Interrupt
Instruction Received
E BPE (E xtended Byte Parity
E nable) (SY M53C875N only)
IID (Illegal Instruction Detected)
Bit 1
Bit 0
RES
7
MDPE
6
BF
5
ABRT
4
SSI
3
SIR
2
RES
1
IID
0
Default>>>
X
0
0
0
0
0
X
0