UPD17P136ACT datasheet(43/292 Pages) NEC

43 / 292 page

CHAPTER 4 PROGRAM MEMORY (ROM)

4.2 PROGRAM MEMORY USAGE

Program memory has the following two main functions:

(1) Storage of the program

(2) Storage of constant data

The program is made up of the instructions which operate the CPU (Central Processing Unit). The CPU executes

sequential processing according to the instructions stored in the program. In other words, the CPU reads each

instruction in the order stored by the program in program memory and executes it.

Since all instructions are 16-bit long words, each instruction is stored in a single address in program memory.

Constant data, such as display patterns, are set beforehand. The MOVT is used for reading constant data in

program memory to transfer data from program memory to the data buffer (DBF) in data memory. Reading the constant

data in program memory is called table reference.

Program memory is read-only (ROM: Read Only Memory) and therefore cannot be changed by any instructions.

4.2.1 Flow of the Program

The program is usually stored in program memory starting from address 0000H and executed sequentially one

address at a time. However, if for some reason a different kind of program is to be executed, it will be necessary to

change the flow of the program. In this case, the branch instruction (BR instruction) is used.

If the same program code is going to appear in a number of places, reproducing the code each time it needs to

be used will decrease the efficiency of the program. In this case, the program should be stored in only one place in

memory. Then, by using the CALL instruction, call the same program. Such a program is called a subroutine. As

opposed to a subroutine, code used during normal operation is called the main routine.

For cases completely unrelated to the flow of the program (in which a section of code is to be executed when a

certain condition arises), the interrupt function is used. Whenever a condition arises that is unrelated to the flow of

the program, the interrupt function can be used to branch the program to a prechosen memory location (called a vector

address).

Items (1) to (5) explain branching of the program using the interrupt function and instructions.

(1) Vector address

Table 4-2 shows the address to which the program is branched (vector address) when a reset or interrupt

occurs.

Table 4-2. Vector Address for the

µPD17134A Subseries

Vector address

Cause of the interrupt

0000H

Reset

0001H

Serial interface interrupt

0002H

Basic interval timer interrupt

0003H

Timer 1 interrupt

0004H

Timer 0 interrupt

0005H

External (INT) interrupt

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