NEC switch - Manual (Page 48)

CHAPTER 3 CPU ARCHITECTURE
User’s Manual U12978EJ3V0UD
48
3.3 Instruction Address Addressing
An instruction address is determined by program counter (PC) contents. 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 78K/0S Series Instruction User’s
Manual (U11047E)).
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.
This means that information is relatively branched to a location between −128 and +127, from the start address
of the next instruction when relative addressing is used.
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.
... PC is the start address of
the next instruction of
a BR instruction.
When S = 1, α indicates all bits 1.