JPH06103478B2 - Micro computer - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a microcomputer, and more particularly to a circuit that prohibits reading of internal memory.

2. Description of the Related Art With the expansion of application fields of single-chip microcomputers, the number of steps of instruction programs tends to increase. Therefore, the capacity of a read-only memory (ROM) for storing instructions and the like to be integrated on the same chip has increased. However, the capacity of ROM that can be integrated on the same chip is limited. 2. Description of the Related Art Conventionally, there is a single-chip microcomputer capable of expanding a memory outside the chip in case the ROM capacity inside the chip is insufficient. As a way to expand the memory outside the chip,
Conceptually, there are two types. FIG. 2 and FIG. 3 are schematic diagrams showing two concepts related to the arrangement of each expansion memory in comparison. Each figure shows the total address space
64K bytes, the capacity of the internal ROM is 8K bytes,
In the figure, 1 is an internal ROM area, and 2 and 3 are external memory areas. In FIG. 2, the address area [4] assigned to the internal ROM 1 is also expanded to the external memory 3, and the internal ROM is not used, and all can be operated by the external memory. In FIG. 3, the address area [4] assigned to the internal ROM 1 is operated by using both the internal ROM 1 and the external memory 2 without expanding it to the outside.

In recent years, unauthorized duplication of programs has become a problem, and program developers are required to take measures to prevent duplication. However, as shown in FIG. 2 above, when all the memory for storing instructions and the like is expanded to the outside, the contents of the program can be easily known, so the necessary programs are stored in the internal ROM, Sometimes the entire program cannot be known only by the program stored in the external memory. However, in FIG. 3 described above, the internal ROM 1 and the external memory 2 are in a continuous area, and there is no distinction in their use. be able to. Therefore, in the operation, the first mode in which the single-chip microcomputer originally operates using only the internal ROM, and the second mode in which only the extended external memory operates as shown in FIG. A method of using both the program stored in the internal ROM and the program stored in the external memory while switching between modes may be used. With this method, it is difficult to know the contents of the internal ROM by an externally given instruction because the internal ROM cannot be accessed when operating using the extended external memory.

In order to improve the operating speed of the microcomputer,
In general, instructions are prefetched, and a method of storing them in a dedicated buffer may be used.

FIG. 4 shows a block diagram of a microcomputer using the above method. Reference numeral 1 is an internal ROM, 2 is an external memory, 4 is a multiplexer, 5 is an instruction prefetch buffer, 6 is an instruction decoder, and 7 is a mode switching control circuit.

Problems to be Solved by the Invention In a microcomputer using such a method, a first mode in which only the internal ROM described above is used for operation, and
Even if the method of switching and operating the second mode using only the external memory is used, at the time of switching from the second mode to the first mode, the instruction prefetched from the external memory 2 has already been read. The instruction in the prefetch buffer is valid even after it is stored in the instruction prefetch buffer 5 and changed to the first mode in which only the internal ROM 1 is used for operation. Therefore, it is possible to read the contents of the internal ROM by using an instruction such as a table lookup function.

An object of the present invention is to solve the above-mentioned problems and to provide a microcomputer in which the contents of the internal ROM cannot be read by an external command.

Means for Solving the Problems In order to solve the above-mentioned problems, the present invention switches between a first mode operating using an internal memory and a second mode operating using an external memory. A second mode in which the external memory is used, and a first mode in which the internal memory is used are provided with a enabling unit and a buffer for prefetching instructions stored in the internal memory or the external memory. It is a microcomputer having a function of setting a specific instruction code in the buffer when switched.

Effect According to the present invention, it becomes impossible to access the internal memory by an external instruction, so that it is possible to realize a microcomputer in which the instruction content of the internal memory cannot be known.

Embodiment FIG. 1 shows a block diagram of an embodiment of the present invention.

In FIG. 1, the mode switching control circuit 7 has an internal function depending on, for example, the state of the input terminal and the state of the control flag.
It has a function of switching between a mode operating using only the ROM 1 and a mode operating using only the external memory 2 extended to the outside. In the case of a mode in which only externally expanded memory is used for operation, as shown in FIG.
The address area of ROM1 is also expanded to the outside, and internal ROM1 is not accessed.

First, the mode that operates using only the internal ROM 1 will be described.

In this case, the multiplexer 4 selects the internal ROM 1. Prior to the execution of the instruction, the instruction code is read from the internal ROM 1 and stored in the instruction prefetch buffer 5. When the instruction is executed, the pre-read instruction code is
The instructions are sequentially read from the instruction prefetch buffer 5, the instruction decoder 6 decodes the instruction code, and a control signal is sent to each part of the microcomputer.

In the mode in which only the external memory 2 is used for operation, the multiplexer 4 selects the external memory 2. Also in this case, as in the case described above, the instruction code read from the external memory 2 is stored in the instruction prefetch buffer 5 prior to the execution of the instruction.

Here, when the mode switching control circuit 7 detects that the mode in which only the external memory 2 is used is changed to the mode in which only the internal ROM 1 is used, the multiplexer 4 is switched to the switching control signal line 8. Sends a control signal to drive the multiplexer 4 from the external memory 2 side to the internal ROM 1
Switch to the side. Further, a control signal for setting a specific instruction such as NOP instruction is output to the instruction prefetch buffer 5 through the instruction setting control signal line 9. By the signal of the instruction setting control signal line 9, the contents of all the instruction prefetch buffers 5 are set to a specific instruction code such as NOP instruction,
The instruction code that has been prefetched from the external memory 2 prior to the instruction and stored in the instruction prefetch buffer 5 becomes invalid.

As described above, according to the present invention, when the internal ROM can be accessed, the instruction code input from the outside becomes invalid, so that the instruction cannot be input from the outside to read the contents of the internal ROM. , The program stored in the internal ROM can be protected.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG.
FIG. 3 is a conceptual diagram when the memory is expanded outside the chip,
FIG. 4 is a block diagram of a conventional example. 1 ... internal ROM, 2 ... external memory, 3 ... external memory having the same address space as internal ROM, 4 ... multiplexer,
5 ... Instruction prefetch buffer, 6 ... Instruction decoder, 7 ...
... Mode switching control circuit, 8 ... Switching control signal line, 9 ...
... Command setting control signal line.

Claims (1)

[Claims] 1. Means for switching between a first mode operating using an internal memory and a second mode operating using an external memory, and the means stored in the internal memory or the external memory. It has a buffer for prefetching instructions, and has a function of setting a specific instruction code in the buffer when switching from the second mode using the external memory to the first mode using the internal memory. Microcomputer.