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Chapter 5
Bus Control Function
User’s Manual U16702EE3V2UD00
5.8 Bus
Priority
Bus hold, instruction fetch (branch), instruction fetch (successive), and operand data accesses are
executed in the external bus cycle.
Bus hold has the highest priority, followed by operand data access, instruction fetch (branch), and
instruction fetch (successive).
An instruction fetch may be inserted between the read access and write access in a read-modify-write
access.
If an instruction is executed for two or more accesses, an instruction fetch and bus hold are not inserted
between accesses due to bus size limitations.
5.9 Boundary
Operation
Conditions
5.9.1 Program
space
(1)
If a branch instruction exists at the upper limit of the internal RAM area, a prefetch operation
straddling over the internal peripheral I/O area (invalid fetch) does not occur.
(2)
Instruction execution to the external memory area cannot be continued without a branch from the
internal ROM area to the external memory area.
5.9.2 Data
space
The V850E/RS1 has an address misalign function.
With this function, data can be placed at all addresses, regardless of the format of the data (word data
or halfword data). However, if the word data or halfword data is not aligned at the boundary, a bus cycle
is generated at least twice, causing the bus efficiency to drop.
(1)
Halfword-length data access
A byte-length bus cycle is generated twice if the least significant bit of the address is 1.
(2)
Word-length data access
(a) A byte-length bus cycle, halfword-length bus cycle, and byte-length bus cycle are generated in
that order if the least significant bit of the address is 1.
(b) A halfword-length bus cycle is generated twice if the lower 2 bits of the address are 10.
Table 5-4:
Bus Priority
Priority
External Bus Cycle
Bus Master
High
Low
Bus hold
External device
DMA transfer
DMAC
Operand data access
CPU
Instruction fetch (branch)
CPU
Instruction fetch (successive)
CPU
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