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Software Developer’s Manual 223
Register Descriptions
13.3 PCI-X Register Access Split
1
The PCI-X specification states that accesses to internal device memory spaces must complete
within a specific target initial latency, or else the device should signal that it completes the
transaction later using a split-completion operation. Due to internal access latencies, read accesses
to most device registers in the Ethernet controller exceeds target initial-access latencies, and
therefore are split.
Once a register read operation has been split, the device may, as part of normal operation, initiate a
large inbound or outbound transmit or receive data burst transaction. The split completion for the
pending register read might be forced to wait until the data burst completes. Therefore, the read
access-delay for most registers can be indeterminate (although is generally bounded by the nature
or normal burst transactions).
A small subset of the internal register space has been identified as most critical for high-perfor-
mance driver execution. The variable completion delay for access to some registers could
potentially limit the performance of such critical routines as Interrupt Service Routines (ISRs). To
help minimize potential critical routine performance, read accesses to a small subset of internal
register space will instead complete without being split. These registers are listed as follows:
1. Not applicable to the 82540EP/EM, 82541xx, or 82547GI/EI.
Category Offset Abbreviation Name
General 00000h CTRL Device Control Register
General 00008h STATUS Device Status Register
General 00010h EECD EEPROM/Flash Control/Data Register
General 00018h CTRL_EXT Extended Device Control Register
General 00020h MDIC MDI Control Register
General 00028h FCAL Flow Control Address Low
General 0002Ch FCAH Flow Control Address High
General 00030h FCT Flow Control Type
General 00038h VET VLAN Ether Type
General 00170h FCTTV Flow Control Transmit Timer Value
General 00178h TXCW Transmit Configuration Word
General 00180h RXCW Receive Configuration Word
General 01000h PBA Packet Buffer Allocation
Interrupt 000C0h ICR Interrupt Cause Read
Interrupt 000C8h ICS Interrupt Cause Set