D17133CS datasheet(39/289 Pages) NEC

39 / 289 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 location in program memory.

Constant data, such as display output patterns, are set beforehand. The MOVT instruction is used 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 memory location 0000H and executed sequentially

one memory location 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 section of 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, this section of program code should

be stored in only one place in memory. Then, by using the CALL instruction, this piece of code can be executed

or read as many times as needed within the program. Such a piece of code 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 CPU instructions.

(1) Vector Address

Table 4-1 shows the address to which the program is branched (vector address) when a reset or interrupt

occurs.

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