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CHAPTER 2 CPU
2.9 Prefix Codes
Placing a prefix code before an instruction partially changes the operation of the
instruction. Three types of prefix codes can be used: bank select prefix, common
register bank prefix, and flag change disable prefix.
■ Bank Select Prefix
The memory space used for accessing data is determined for each addressing mode.
When a bank select prefix is placed before an instruction, the memory space used for accessing data by that
instruction can be selected regardless of the addressing mode.
Table 2.9-1 lists the bank select prefixes and the corresponding memory spaces.
Use the following instructions with care:
●
String instructions (MOVS, MOVSW, SCEQ, SCWEQ, FILS, FILSW)
The bank register specified by an operand is used regardless of the prefix.
●
Stack manipulation instructions (PUSHW, POPW)
SSB or USB is used according to the S flag regardless of the prefix.
●
I/O access instructions
MOVA,io/MOVio,A/MOVXA,io/MOVWA,io/MOVWio,A/MOVio,#imm8
MOVWio,#imm16/MOVBA,io:bp/MOVBio:bp,A/SETBio:bp/CLRBio:bp
BBCio:bp,rel/BBSio:bp,rel/WBTC,WBTS
The IO space of the bank is used regardless of the prefix.
●
Flag change instructions (AND CCR,#imm8, OR CCR,#imm8)
The instruction is executed normally, but the prefix affects the next instruction.
●
POPW PS
SSB or USB is used according to the S flag regardless of the prefix. The prefix affects the next instruction.
Table 2.9-1 Bank Select Prefix
Bank select prefix Selected space
PCB PC space
DTB Data space
ADB AD space
SPB Either the SSP or USP space is used according to the stack flag value.