
CHAPTER 3 CPU ARCHITECTURE
47
3.3 Instruction Address Addressing
An instruction address is determined by program counter (PC) contents. The PC content is 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 Series User's Manual:
Instructions (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. In other words,
execution branches in a relative range of –128 to +127 from the start address of the subsequent instruction in the
relative addressing mode.
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.
...