Dma byte counter (dbc) – Avago Technologies LSI53C896 User Manual
Page 177
SCSI Registers
4-65
Version 3.3
Copyright © 1998–2003 by LSI Logic Corporation. All rights reserved.
Registers: 0x24–0x26
DMA Byte Counter (DBC)
Read/Write
DBC
DMA Byte Counter
[23:0]
This 24-bit register determines the number of bytes
transferred in a Block Move instruction. While sending
data to the SCSI bus, the counter decrements as data is
moved into the DMA FIFO from memory. While receiving
data from the SCSI bus, the counter decrements as data
is written to memory from the LSI53C896 SCSI function.
The DBC counter decrements each time data is
transferred on the PCI bus. It is decremented by an
amount equal to the number of bytes that are transferred.
The maximum number of bytes that can be transferred in
any one Block Move command is 16,777,215 bytes. The
maximum value that can be loaded into the
register is 0xFFFFFF. If the
instruction is a Block Move and a value of 0x000000 is
loaded into the DBC register, an illegal instruction
interrupt occurs if the LSI53C896 SCSI function is not in
the target mode, Command phase.
The
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 refer to
Chapter 5, “SCSI SCRIPTS Instruction Set.”
The
power-up value of this register is indeterminate.
23
0
DBC
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x