UPD78044F datasheet(70/472 Pages) NEC

70 / 472 page

Chapter 3

CPU Architecture

User’s Manual U16504EE1V1UD00

3.3 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. However, 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 User's Manual - Instructions (U12326E).

3.3.1 Relative addressing

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.

Figure 3-15:

Relative Addressing

876

jdisp8

When S = 0, all bits of a are 0.

When S = 1, all bits of a are 1.

PC indicates the start address

of the instruction

after the BR instruction.

...

UPD78044F Datasheet(PDF) 70 Page - NEC