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CHAPTER 2 CPU
2.4
Prefix Codes
By inserting a prefix code before an instruction, part of an instruction operation may
change. Three types of prefix codes are provided: bank select prefixes, common
register bank prefixes, and flag change suppress prefixes.
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Bank select prefix (PCB, DTB, ADB, SPB)
Memory space used in data access is determined according to the addressing mode.
By inserting a bank select prefix before an instruction, the instruction selects the memory space
used for data access regardless of the addressing mode in use.
Table 2.4-1 shows the relationship between the bank select prefix and a selected space.
Table 2.4-1 Bank select prefix
Bank select prefix
Be careful when you are using the following instructions:
❍ String instruction (MOVS, MOVSW, SCEQ, SCWEQ, FILS, FILSW)
This uses bank registers specified with an operand regardless of a prefix.
❍ Stack operation instruction (PUSHW, POPW)
SSB or USB is used depending on the S-flag, regardless of a prefix.
❍ I/O access instruction
MOVA A, io/MOV io, A/MOVX A, io/MOVW A, io/MOVW io, A
MOV io, #imm8/MOVW io, #imm8/MOBV A, io: bp/MOVB io: bp, A
SETB io: bp/CLRB io: bp/BBC io:bp, rel/BBS io:bp, rel WBTC
WBTS
The I/O space of a bank is used regardless of whether a prefix is in an instruction.
❍ Flag change instruction (AND CCR,#imm8,OR CCR,#imm8)
An instruction operation is normal, but a prefix affects the next instruction.
❍ POPW PS
Regardless of prefix, SSB or USB is used depending on the S-flag. A prefix affects the next
instruction.
❍ MOV ILM, #imm8
If an instruction operation is normal, but a prefix affects the next instruction.
40
PCB
DTB
ADB
SPB
Either SSB or USB space is used depending on the stack flag value.
Selected space
PC space
Data space
AD space
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