Table of Contents
Advertisement
ARM DDI 0337B
Pushed operations improve performance where writes might be faster than reads. They
are used as part of in-line tests, for example Flash ROM programming and monitor
communication. Pushed operations are enabled using the Transaction Mode bits,
TRNMODE, in the DP Control/Status Register, see The Control/Status Register,
CTRL/STAT on page 12-53.
Considering pushed operations on a specific AP makes it easier to understand how these
operations are implemented. On an AHB-AP, if you perform an AP write transaction to
the Data Read/Write (DRW) Register, or to one of the Banked Data (BD0 to BD3)
Registers, with either pushed compare or pushed verify active:
•
The DP holds the data value from the AP write transaction in the pushed compare
logic, see Figure 12-19 on page 12-44.
•
The AP reads from the address indicated by the AP Transfer Address Register
(TAR), see AHB-AP Transfer Address Register on page 11-38.
•
The value returned by this read is compared with the value held in the pushed
compare logic, and the STICKYCMP bit is set depending on the result. The
comparison is masked as required by the MASKLANE bits. For more
information see The Control/Status Register, CTRL/STAT on page 12-53.
As described, whenever an AP write transaction is performed with pushed compare or
pushed verify active, the actual AP access that results is a read operation, not a write.
Note
Performing an AP read transaction with pushed compare or pushed verify active causes
Unpredictable behavior.
On a Serial Wire DP, performing an AP read transaction with pushed compare or pushed
verify active returns a value. This means the wire-level protocol remains coherent.
However, the value returned is Unpredictable, and the read has Unpredictable
side-effects.
Example use of pushed verify operation on a AHB-AP
You can use pushed verify to verify the contents of system memory.
•
Make sure that the AHB-AP Control/Status Word (CSW) is set up to increment
the Transfer Address Register (TAR) after each access. See The Control/Status
Register, CTRL/STAT on page 12-53.
•
Write to the Transfer Address Register (TAR) to indicate the start address of the
Debug Register region that is to be verified, see AHB-AP Transfer Address
Register on page 11-38.
Copyright © 2005, 2006 ARM Limited. All rights reserved.
Debug Port
12-45
Advertisement
Table of Contents
