NEC UPD789026 Series, User Manual

Page: 60 / 213

CHAPTER 3 CPU ARCHITECTURE
User's Manual U11919EJ3V0UM00
60
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 User's Manual

 

Instruction (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. 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 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".
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