CHAPTER 3 CPU ARCHITECTURE
Preliminary User’s Manual U16846EJ1V0UD
52
3.3 Instruction Address Addressing
An instruction address is determined by program counter (PC) contents and 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 Instructions User’s Manual (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, relative addressing consists of relative branching from the start address of the following
instruction to the
−
128 to +127 range.
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 are 0.
When S = 1, all bits of are 1.
PC indicates the start address
of the instruction after the BR instruction.
...
α
α
