
SCSI Operating Registers
SYM53C825A/825AE Data Manual
5-31
Register 23 (A3)
Chip Test Six (CT EST 6)
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 CT EST 4
register. Reading this register unloads data
from the appropriate byte lane of the DMA
FIFO as determined by the FBL bits in the
CT EST 4 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 pre-
vent DMA data from being corrupted, this reg-
ister should not be accessed before starting or
restarting SCRIPT S operation. T his register
should only be written when testing the DMA
FIFO using the CT EST 4 register. Writes to
this register while the test mode is not enabled
will have unexpected results.
Registers 24-26 (A4-A6)
DMA Byte Counter (DBC)
Read/Write
T his 24-bit register determines the number of bytes
to be 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 SYM53C825A. T he DBC
counter is decremented each time that data is trans-
ferred on the PCI bus. It is decremented by an
amount equal to the number of bytes that were
transferred.
T he maximum number of bytes that can be trans-
ferred in any one Block Move command is
16,777,215 bytes. T he 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 illegal instruc-
tion interrupt will occur if the SYM53C825A is not
in target mode, Command phase.
T he DBC register is also used to hold the least sig-
nificant 24 bits of the first dword of a SCRIPT
fetch, and to hold the offset value during table indi-
rect I/O SCRIPT S. For a complete description, see
Chapter Six, “Instruction Set of the I/O Proces-
sor.” T he power-up value of this register is indeter-
minate.
DF7
7
DF6
6
DF5
5
DF4
4
DF3
3
DF2
2
DF1
1
DF0
0
Default>>>
0
0
0
0
0
0
0
0