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Level Two Interface
ARM DDI 0363E Copyright © 2009 ARM Limited. All rights reserved. 9-4
ID013010 Non-Confidential, Unrestricted Access
9.2.1 Identifiers for AXI bus accesses
Accesses on the AXI bus use ID values as follows:
Outstanding write/read access on different IDs
This means, for example, that a Non-cacheable (NC) read and linefills can be
outstanding on the AXI bus simultaneously as long as the IDs are different.
At the same time, there can be:
• up to seven outstanding reads, each with one of seven different ID values,
that consists of:
— a data side read NC access, RID0
— an instruction side read NC access or an instruction side read
Cacheable access, RID1
— five outstanding data side linefills on the AXI bus, RID3 - RID7.
• up to two IDs on outstanding writes, that consist of:
— single or burst NC writes or write-through (WT) writes, WID0
— evictions, WID1.
Outstanding write accesses with the same ID
When the address and data of the first write are both put on AXI bus, another write
request with same ID can be sent when the address or data channel is released.
For example, the new address can be sent with the same ID, before the target
accepts the data of the first write.
Note
• The AXI master does not generate two outstanding read accesses with the same ID.
• The AXI master does not interleave write data from two different bursts, even if the bursts
have different IDs.
9.2.2 Write response
The AXI master requires that the slave does not return a write response until it has received both
the write data and the write address.
9.2.3 Linefill buffers and the AXI master interface
On the data side there are two LineFill Buffers (LFBs), LFB0 and LFB1. Each request from the
data cache controller or from the STore Buffer (STB) can be allocated to either LFB0 or LFB1.
On the instruction side, there is one LFB. This is the Instruction LFB (ILFB), that treats
instruction linefill requests or Non-cacheable instruction reads in the same way.
The linefill buffers:
• get returned data from the AXI bus for linefill requests
• get returned data from the AXI bus for any Non-cacheable
LDR
or
LDM
s
• get data from the STB to write as a burst on the AXI bus (LFB0 and LFB1 only).
Single writes do not use LFBs.
The LFBs are 256 bits wide so that an entire cache line can be written to the cache RAMs in one
cycle. While the LFB is being filled from L2 memory, its bytes can be merged with write data
from the STB.