NEC 78K/0 Series - Manual (Page 20)

20 User's Manual U12326EJ4V0UM
CHAPTER 3 ADDRESSING
3.1 Instruction Address Addressing
An instruction address is determined by program counter (PC) contents. The PC 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 each instruction, refer to CHAPTER 5
EXPLANATION OF INSTRUCTIONS).
3.1.1 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, in relative addressing, the value is relatively transferred to the range between –128 and
+127 from the start address of the following instruction.
This function is carried out when the “BR $addr16” instruction or a conditional branch instruction is executed.
[Illustration]
15 0
PC
15 0
S
15 0
PC
+
8 7 6
α
jdisp8
When S = 0, α indicates all bits "0".
When S = 1, α indicates all bits "1".
... PC is the start address of
the next instruction of
a BR instruction.