Texas Instruments TMS320C80, User Manual

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Technique #2: Using Pointers to Allocate RAM
2-9 Programming for Compatibility
each PP. Each PP can then use the RAM without being aware of
whether the RAM was allocated from a ’C80 or a ’C82.
The MP should pass a pointer to this RAM through a command
buffer, such as the one shown in Figure 2–3 (taken from Sec-
tion 5.2.6,
Command Buffer, in the TMS320C80 Multitasking
Executive User’s Guide). The args field in the command buffer
contains a pointer to a buffer in shared memory that contains the
argument values for the command. The MP can use one of the
additional blocks of RAM as an argument buffer and pass the
pointer to the PP through the args field of the command buffer.
Figure 2–3. Structure of a PP Command Buffer
link
flag
function
args
mailbox
Pointer to next command buffer in linked list
Full/not-empty flag
Pointer to command function
Pointer to buffer containing argument values
Pointer to server PP’s mailbox
32 bits
msgValue
intCode
Message to put in mailbox for client
Code for message interrupt to client
2.4.4 Considerations When Using Technique #2
In the case of the ’C82, the additional RAM is allocated from the
local RAM of PP0 and PP1. The PPs can access this RAM over
the local-bus connections of crossbar. For the ’C80, however, the
additional RAM may not be local to the PP that accesses the
RAM. If a PP attempts to access a nonlocal RAM through its local
port, however, the only impact is that the access is delayed for
one clock cycle. This delay may slightly alter the execution time
of the software, but is otherwise transparent to software.