
SYM53C876/876E Data Manual
4-45
Registers
SCSI Registers
Register 23h
Chip Test Six (CTEST6)
Read/Write
Bits 7-0 DF7-DF0 (DMA FIFO)
Writing to this register writes data to the
appropriate byte lane of the DMA FIFO as
determined by the FBL bits in the CTEST4
register. Reading this register unloads data
from the appropriate byte lane of the DMA
FIFO as determined by the FBL bits in the
CTEST4 register. Data written to the FIFO is
loaded into the top of the FIFO. Data read
out of the FIFO is taken from the bottom. To
prevent DMA data from being corrupted, this
register should not be accessed before starting
or restarting SCRIPTS operation. Write this
register only when testing the DMA FIFO
using the CTEST4 register. Writing to this
register while the test mode is not enabled
produces unexpected results.
Registers 24h-26h
DMA Byte Counter (DBC)
Read/Write
This 24-bit register determines the number of
bytes transferred in a Block Move instruction.
While sending data to the SCSI bus, the counter
is decremented as data is moved into the DMA
FIFO from memory. While receiving data from
the SCSI bus, the counter is decremented as data
is written to memory from the SYM53C876 SCSI
function. The DBC counter is decremented each
time data is transferred on the PCI bus. It is dec-
remented by an amount equal to the number of
bytes that are transferred.
The maximum number of bytes that can be trans-
ferred in any one Block Move command is
16,777,215 bytes. The maximum value that can
be loaded into the DBC register is FFFFFFh. If
the instruction is a Block Move and a value of
000000h is loaded into the DBC register, an ille-
gal instruction interrupt occurs if the
SYM53C876 SCSI function is not in target
mode, Command phase.
The DBC register also holds the least significant
24 bits of the first dword of a SCRIPTS fetch,
and to hold the offset value during table indirect I/
O SCRIPTS. For a complete description see
Chapter 5,
SCSI SCRIPTS Instruction Set
.The
power-up value of this register is indeterminate.
DF7
7
Default >>>
0
DF6
6
DF5
5
DF4
4
DF3
3
DF2
2
DF1
1
DF0
0
0
0
0
0
0
0
0