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CHAPTER 3 CPU
3.9.1
Bank Select Prefix (PCB, DTB, ADB, SPB)
A bank select prefix is placed before an instruction to select the memory space
accessed by the instruction regardless of the addressing method.
■ Bank Select Prefixes (PCB, DTB, ADB, SPB)
The memory space used for data access is defined for each addressing method. If a bank select prefix is
placed before an instruction, the memory space accessed by the instruction can be selected regardless of the
addressing method. Table 3.9-1 lists the bank select prefix codes and selected memory spaces.
Table 3.9-1 Bank Select Prefix Codes and selected Memory Spaces
Bank select prefix
If a bank select prefix is used, some instructions perform an unexpected operation.
Table 3.9-2 lists the instructions that are not affected by bank select prefix codes. Table 3.9-3 lists
instructions that require caution when they are used.
Table 3.9-2 Instructions not affected by Bank Select Prefix Codes
Instruction type
String instruction
Stack
operation
I/O access instruction
Interrupt return
instruction
58
PCB
Program space
DTB
Data space
ADB
Additional space
When the value of the S flag in the condition code register (CCR) is "0" and the
SPB
user stack space is "1", the system stack space is used.
Instruction
MOVS
MOVSW
SCEQ
SCWEQ
FIL
SFILSW
PUSHW
POPW
MOV
A
MOVW
A, io
MOV
io, A
MOV
io, #imm8
MOVB
A, io : bp
SETB
io : bp
BBC
io : bp, rel
WBTC
io, bp
RETI
Selected space
The bank register specified by the
operand is used whether or not a prefix is
used.
When the S flag is "0", the user stack bank
(USB) is used whether or not there is a
prefix. When the S flag is "1", the system
stack bank (SSB) is used regardless of
whether a prefix is used.
MOVX
A, io
MOVW io, A
The I/O space (000000
MOVW io,#imm16
accessed whether or not there is a
MOVB
io : bp, A
prefix.
CLRB
io : bp
BBS
io : bp, rel
WBTS
io : bp
The system stack bank (SSB) is used
whether or not a prefix is used.
Effect of bank select prefix
to 0000FF
H
) is
H
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