MIPS MIPS32 74K Series, Programming Manual

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8.1 EJTAG on-chip debug unit
Programming the MIPS32® 74K™ Core Family, Revision 02.14 106
• dseg: is the whole debug-mode-only memory area.
It’s possible for debug-mode software to read the “kseg2”-mapped locations “underneath” by setting
Debug[LSNM] (see Figure 8.1 ).
• dmseg: is the memory region where reads and writes are implemented by the probe. But if no active probe is
plugged in, or if
DCR[PE] is clear, then accesses here cause reads and writes to be handled like regular “kseg3”
accesses.
• drseg: is where the debug unit’s main register banks are accessed. Accesses to “drseg” don’t go off core. Regis-
ters in “drseg” are word-wide, and should be accessed only with 32-bit word-wide loads and stores.
• fast-talk: is a corner of “dmseg” where probe-mapped reads and writes use a more JTAG-efficient block-mode
probe protocol, reducing the amount of JTAG traffic and allowing for faster data transfer. There’s no details about
how it’s done in this document, see
[EJTAG] .
• debug entry: is the debug exception entry point. Because it lies in “dmseg”, the debug code can be implemented
wholly in probe memory, allowing you to debug a system which has no physical memory reserved for debug.
• TCB Registers : These are the PDtrace EJTag Registers. They are physically located in the PDtrace unit, and
managed by the PDtrace unit. For software to access the on-chip trace memory, these registers are mapped to
drseg.
8.1.6 EJTAG CP0 registers, particularly Debug
In normal circumstances (specifically, when not in debug mode), the only software-visible part of the debug unit is its
set of three CP0 registers:
• Debug which has configuration and control bits, and is detailed below;
• DEPC keeps the restart address from the last debug exception (automatically used by the
deret
instruction);
• DESAVE is a CP0 register which is just 32-bits of read/write space. It’s available for a debug exception handler
which needs to save the value of a first general-purpose register, so that it can use that register as an address base
to save all the others.
Debug is the most complicated and interesting. It has so many fields defined that we’ve taken them in three groups:
debug exception cause bits in Figure 8.2 , information about regular exceptions which want to happen but can’t
because you’re in debug mode in Figure 8.3 , and everything else. The "everything else" category includes the most
important fields and comes first, in Figure 8.1 .