IDT EJTAG System
EJTAG Processor Core Extensions
79RC32438 User Reference Manual
20 - 10
November 4, 2002
Notes
There are no timing requirements with respect to transactions to dmseg, which the probe services.
Therefore, a system watchdog must be disabled during dseg transactions, so accesses can take any
amount of time without being terminated. The protocol for accesses to dmseg does not allow a transaction
to be aborted once started, except by a reset or soft reset. Transactions of all sizes are allowed to dmseg.
Merging is allowed for accesses to dmseg, whereby for example two byte accesses can be merged to
one halfword access, and debug software is thus required to allow merging. However, merging must only
occur for accesses which can be combined into legal processors accesses, since processor access can
only indicate accesses which can occur due to a single load/store, thus not for example accesses to only
first and last bytes of a word. The SYNC instruction, followed by appropriate spacing, can be executed to
ensure that earlier accesses to dmseg are committed thus will not be merged with later accesses.
The processor can do speculative fetching from dmseg whereby it can fetch doublewords even if an
instruction that is not required in the execution flow is thereby fetched. For example, if the DERET instruc-
tion is fetched as the first word of a doubleword, the instruction in the second word is not executed. For
details, refer to architecture description covering speculative fetching from uncached area in general.
If the TAP is not present in the implementation, the operation of the processor is UNDEFINED if the
dmseg is accessed.
Access to drseg (EJTAG Registers) Address Range
Note:
Instruction fetches from drseg are not allowed. The operation of the processor is
UNDEFINED if the processor tries to fetch from drseg.
When the NoDCR bit is 0 in the Debug register, it indicates that the processor is allowed to access the
entire drseg segment and can therefore respond to all transactions to drseg.
The DCR register, at offset 0x0000 in drseg, is always available if dseg is present. Debug software is
expected to read the DCR register to determine what other memory-mapped registers exist in drseg. The
value returned in response to a read of any un-implemented memory-mapped register is UNPREDICT-
ABLE, and writes are ignored to any un-implemented register in drseg. The allowed transaction size is
limited for drseg. Only word size transactions are allowed for 32-bit processors, and only doubleword size
transactions are allowed for 64-bit processors. Operation of the processor is UNDEFINED for other transac-
tion sizes.
Debug Mode Handling of Processor Resources
Unless otherwise specified, the processor resources in Debug Mode are handled identically to those in
Kernel Mode. Some identical cases are described in the following sections for emphasis. In addition, see
the following related sections for more information:
“Debug Mode Exceptions” on page 20-19 covering exception handling in Debug Mode.
“Interrupts and NMIs” on page 20-21 for handling in both Debug and Non-Debug Modes.
“Reset and Soft Reset of Processor” on page 20-22 for handling in both Debug and Non-Debug Modes.
NoDCR
bit in
Debug
Register
Trans-
action
LSNM bit in
Debug
Register
Access
1
x
1
1.
x = don’t care
0 (read only)
Kernel Mode address space
Fetch
x
Operation of the processor is UNDE-
FINED at fetch.
Load/Store
0
drseg
1
Kernel Mode address space
Table 20.9 Access to drseg Address Range