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CHAPTER 3 CPU ARCHITECTURE
User’s Manual U15104EJ2V0UD
3.3 Instruction Address Addressing
An instruction address is determined by program counter (PC) contents, and the contents are normally incremented
(+1 for each byte) automatically according to the number of bytes of an instruction to be fetched each time another
instruction is executed. When a branch instruction is executed, the branch destination information is set to the PC
and branched by the following addressing. (For details of instructions, refer to 78K/0 User’s Manual Instruction
(U12326E)).
3.3.1 Relative addressing
[Function]
The value obtained by adding 8-bit immediate data (displacement value: jdisp8) of an instruction code to the
start address of the following instruction is transferred to the program counter (PC) and branched. The
displacement value is treated as signed two's complement data (–128 to +127) and bit 7 becomes a sign bit. That
is, using relative addressing, the program branches in the range –128 to +127 relative to the first address of the
next instruction. This function is carried out when the BR $addr16 instruction or a conditional branch instruction
is executed.
[Illustration]
15
0
PC
+
15
0
8
7
6
S
15
0
PC
α
jdisp8
When S = 0, all bits of
α
are 0.
When S = 1, all bits of
α
are 1.
PC indicates the start address
of the instruction
after the BR instruction.
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