Instruction Address Addressing; Relative Addressing - NEC PD789860 User Manual

4.3 Instruction Address Addressing

An instruction address is determined by the 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.
information is set to the PC to branch by the following addressing (for details of each instruction, refer to 78K/0S
Series Instructions User's Manual (U11047E)).

4.3.1 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) to branch. The displacement
value is treated as signed two's complement data (–128 to +127) and bit 7 becomes the sign bit. In other words,
the range of branch in relative addressing is between –128 and +127 of 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
15
15
PC
When S = 0, α indicates that all bits are "0".
When S = 1, α indicates that all bits are "1".
CHAPTER 4 CPU ARCHITECTURE
When a branch instruction is executed, the branch destination address
PC
+
8 7 6
α
S
User's Manual U14826EJ5V0UD
0
... PC is the start address of
the next instruction of
a BR instruction.
0
jdisp8
0
49