
2.1 CPU
31
2.1.3 Prefix Codes
Prefix codes may be placed before instructions to partially alter the operation of the instruction. There are
three types of prefixes: bank select prefixes, common register bank prefixes and flag change suppression
prefixes.
s Bank Select Prefixes
The area of memory space used in a data access operation is determined by individual addressing. By
placing a bank select prefix before an instruction, any area of memory space accessed in that instruction
can be specified without regard to the addressing mode. Table 2.1.6 lists bank select prefixes in relation
to the areas of memory space selected.
Table 2.1.6 Bank Select Prefixes
Note that the instructions listed in Table 2.1.7 ignore bank select prefixes, and the bank select prefixes
used with instructions listed in Table 2.1.8 take effect with the following instruction.
Table 2.1.7 Instructions Unaffected by Bank Select Prefixes
Bank select prefix
Area selected
PCB
PC space
DTB
Data space
ADB
AD space
SPB
User stack space when the S flag in
the CCR register is ‘0,’ and system
stack space when this flag is ‘1.’
Instruction type
Instruction
Effect of bank select prefix
String instructions
MOVS
MOVSW
SCEQ
SCWEQ
FILS
FILSW
Instruction uses bank register desig-
nated by operand, regardless of pre-
fix.
Stack operation
instructions
PUSHW
POPW
Instruction uses USB if S-flag is ‘0’
and SSB if S-flag is ‘1’, regardless
of prefix.
I/O access instruc-
tions
MOV
A,io
MOVX
A,io
MOVW A,io
MOV
io,A
MOVW
io,A
MOV
io,#imm8
MOVW
io,#imm16
MOVB A,io:bp
MOVB
io:bp,A
SETB
io:bp
CLRB
io:bp
BBC
io:bp,rel
BBS
io:bp,rel
WBTC
io:bp
WBTS
io:bp
Space 000000H to 0000FFH is
accessed, regardless of prefix.
Interrupt recovery
instructions
RETI
Uses SSB regardless of prefix.