Memory Interface
1-10
Table 1−2 summarizes how these buses are used during accesses.
Table 1−2. Summary of Bus Use During Data-Space and Program-Space Accesses
Access Type
Address Bus
Data Bus
Read from program space PAB
PRDB
Read from data space
DRAB
DRDB
Write to program space PAB DWDB
Write to data space
DWAB DWDB
A program-space read and a program-space write cannot happen simulta-
neously 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 exam-
ple, 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.
1.4.2 Special Bus Operations
Typically, PAB and PRDB are used only for reading instructions from program
space, and DWDB is used only for writing data to data space. However, the
instructions in Table 1−3 are exceptions to this behavior. For more details
about using these instructions, see Chapter 6, Assembly Language Instruc-
tions.
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