152
Am79C973/Am79C975
P R E L I M I N A R Y
CSR82: Transmit Descriptor Address Pointer
Lower
Bit
Name
Description
31-16 RES
Reserved locations. Written as
zeros and read as undefined.
15-0
TXDAPL
Contains the lower 16 bits of the
transmit descriptor address cor-
responding to the last buffer of
the previous transmit frame. If the
previous transmit frame did not
use buffer chaining, then TXDA-
PL contains the lower 16 bits of
the previous frame
’
s transmit de-
scriptor address.
When both the STOP or SPND
bits are cleared, this register is
updated
by
Am79C975 controller immediate-
ly before a transmit descriptor
write.
Am79C973/
Read accessible always. Write
accessible through the PXDAL
bits (CSR60) when the STOP or
SPND bit is set. TXDAPL is set to
0 by H_RESET and are unaffect-
ed by S_RESET or STOP.
CSR84: DMA Address Register Lower
Bit
Name
Description
31-16 RES
Reserved locations. Written as
zeros and read as undefined.
15-0
DMABAL
This register contains the lower
16 bits of the address of system
memory for the current DMA cy-
cle. The Bus Interface Unit con-
trols the Address Register by
issuing increment commands to
increment the memory address
for sequential operations. The
DMABAL register is undefined
until
the
first
Am79C975 controller DMA oper-
ation.
Am79C973/
Read/Write accessible only when
either the STOP or the SPND bit
is set. These bits are unaffected
by H_RESET, S_RESET, or
STOP.
CSR85: DMA Address Register Upper
Bit
Name
Description
31-16 RES
Reserved locations. Written as
zeros and read as undefined.
15-0
DMABAU
This register contains the upper
16 bits of the address of system
memory for the current DMA cy-
cle. The Bus Interface Unit con-
trols the Address Register by
issuing increment commands to
increment the memory address
for sequential operations. The
DMABAU register is undefined
until
the
first
Am79C975 controller DMA oper-
ation.
Am79C973/
Read/Write accessible only when
either the STOP or the SPND bit
is set. These bits are unaffected
by H_RESET, S_RESET, or
STOP.
CSR86: Buffer Byte Counter
Bit
Name
Description
31-16 RES
Reserved locations. Written as
zeros and read as undefined.
15-12 RES
Reserved. Read and written with
ones.
11-0
DMABC
DMA Byte Count Register. Con-
tains the two's complement of the
current size of the remaining
transmit or receive buffer in
bytes. This register is increment-
ed by the Bus Interface Unit. The
DMABC register is undefined un-
til written.
Read/Write accessible only when
either the STOP or the SPND bit
is set. These bits are unaffected
by H_RESET, S_RESET, or
STOP.
CSR88: Chip ID Register Lower
Bit
Name
Description
31-28 VER
Version. This 4-bit pattern is
silicon-revision dependent.
Read accessible only when either
the STOP or the SPND bit is set.