
SCSI Operating Registers
SYM53C875/875E Data Manual
5-35
Register 38 (B8)
DMA Mode (DMODE)
Read/Write
Bit 7-6
T hese bits control the maximum number of
transfers performed per bus ownership, regard-
less of whether the transfers are back-to-back,
burst, or a combination of both. T he
SYM53C875 asserts the Bus Request (REQ/)
output when the DMA FIFO can accommo-
date a transfer of at least one burst size of data.
Bus Request (REQ/) is also asserted during
start-of-transfer and end-of-transfer cleanup
and alignment, even though less than a full
burst of transfers may be performed. T he
SYM53C875 inserts a “fairness delay” of four
CLK s between burst-length transfers (as set in
BL1-0) during normal operation. T he fairness
delay is not inserted during PCI retry cycles.
T his gives the CPU and other bus master
devices the opportunity to access the PCI bus
between bursts.
BL1-BL0 (Burst Length)
Bit 5
SIOM (Source I/O-Memory E nable)
T his 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.
T his function is useful for register-to-memory
operations using the Memory Move instruc-
tion when the SYM53C875 is I/O mapped.
Bits 4 and 5 of the CT EST 2 register can be
used to determine the configuration status of
the SYM53C875.
Bit 4
DIOM (Destination I/O-Memory
E nable)
T his 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.
T his function is useful for memory- to- register
operations using the Memory Move instruc-
tion when the SYM53C875 is I/O mapped.
Bits 4 and 5 of the CT EST 2 register can be
used to determine the configuration status of
the SYM53C875.
Bit 3
E RL (E nable Read Line)
T his bit enables a PCI Read Line command. If
PCI cache mode is enabled by setting bits in
the PCI Cache Line Size register, this chip
issues a Read Line command on all read cycles
if other conditions are met. For more informa-
tion on these conditions, refer to Chapter 3.
Bit 2
E RMP (E nable Read Multiple)
T his bit, when set, will cause Read Multiple
commands to be issued on the PCI bus after
certain conditions have been met. T hese condi-
tions are described in Chapter 3.
BL1
7
BL0
6
SIOM
5
DIOM
4
ER
3
ERMP
2
BOF
1
MAN
0
Default>>>
0
0
0
0
0
0
0
0
BL2
(CTEST5
bit 2)
BL1
BL0
Burst Length
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
2- transfer burst
4- transfer burst
8-transfer burst
16-transfer burst
32-transfer burst*
64-transfer burst*
128-transfer burst*
Reserved
* Only valid of the FIFO size is set to 536 bytes