1.4.1 Address and Data Buses
1.4.2 Alignment of 32-Bit Accesses to Even Addresses
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Memory Interface
Like the C28x, the memory interface has three address buses:
•
PAB: Program address bus
The PAB carries addresses for reads and writes from program space. PAB is a 22-bit bus.
•
DRAB: Data-read address bus
The 32-bit DRAB carries addresses for reads from data space.
•
DWAB: Data-write address bus
The 32-bit DWAB carries addresses for writes to data space.
The memory interface also has three data buses:
•
PRDB: Program-read data bus
The PRDB carries instructions during reads from program space. PRDB is a 32-bit bus.
•
DRDB: Data-read data bus
The DRDB carries data during reads from data space. DRDB is a 32-bit bus.
•
DWDB: Data-/Program-write data bus
The 32-bit DWDB carries data during writes to data space or program space.
A program-space read and a program-space write cannot happen simultaneously because both use the
PAB. Similarly, a program-space write and a data-space write cannot happen simultaneously because
both use the DWDB. Transactions that use different buses can happen simultaneously. For example, the
CPU can read from program space (using PAB and PRDB), read from data space (using DRAB and
DRDB), and write to data space (using DWAB and DWDB) at the same time. This behavior is identical to
the C28x CPU.
The C28x+FPU CPU expects memory wrappers or peripheral-interface logic to align any 32-bit read or
write to an even address. If the address-generation logic generates an odd address, the CPU will begin
reading or writing at the previous even address. This alignment does not affect the address values
generated by the address-generation logic.
Most instruction fetches from program space are performed as 32-bit read operations and are aligned
accordingly. However, alignment of instruction fetches are effectively invisible to a programmer. When
instructions are stored to program space, they do not have to be aligned to even addresses. Instruction
boundaries are decoded within the CPU.
You need to be concerned with alignment when using instructions that perform 32-bit reads from or writes
to data space.
SPRUEO2A – June 2007 – Revised August 2008
Introduction
11
Содержание TMS320C28 series
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