8051 Microcontroller Memory Organization

8051 Microcontroller Memory Organization – The 8051 microcontroller memory is organized into two types: internal and external. Internal memory includes RAM and ROM, and external memory is used for additional storage when the internal RAM or ROM is insufficient.

8051 Internal Memory Management

The 8051 microcontroller has internal RAM and ROM. Additional memory can be added externally using suitable circuits. The RAM is used to store variable data during the execution of the program. The ROM is used to store program code for an application at the time of manufacturing; once written can not be altered.

Internal RAM

The 128-byte internal RAM is shown in the figure below. The internal RAM has three different areas, which are:

1. Register Bank
2. Bit Addressable Range
3. General Purpose Area

1. Register Bank: 32 working registers are arranged as a part of internal RAM in four register banks. These four register banks are numbered from 0 to 3. Each register bank contains eight registers named R0 to R7. Each register can be addressed by its name or by its RAM address. The byte address of each register is shown in the figure above. For example, the address of register R6 in bank 1 is 0EH. Bit RS0 and RS1 in the Program Status Word (PSW) determine the bank of registers that is currently in use at any time when the program is running.
2. Bit Addressable Range: 16 Registers of RAM, with byte addresses from 20H to 2FH, are bit addressable. It means each bit of these 16 registers has its own address, which ranges from 00H to 2FH. These registers can also be addressed by byte address, which ranges from 20H to 2FH. When any information is represented by a single bit, these bit-addressable registers are useful to store the information.
3. General Purpose Area: A general-purpose RAM area above the bit addressable area, from 30H to 7FH, addressable as bytes.

Internal ROM

The internal ROM is used to store program code for the application at the time of manufacturing. The 8051 is organized so that data memory and program code memory can be in two entirely different physical memory entities. Each has the same address range. A corresponding block of internal program code, contained in an internal ROM, occupies code address space 0000H to 0FFFH.

External Memory of 8051

In some applications, internal RAM or ROM is not sufficient, so external memory is required. Another limitation of internal ROM is that once the program is written, it can not be altered. For improvement in programs, additional external memory is connected. Similarly, when a program is in a developing state, it can not be tried with the internal ROM. Since ROM can be programmed only once, it may cost a lot. Keeping these facts in mind, additional external memory is connected with the 8051 microcontroller.

The connections for adding external memory are shown in the figure below. The figure shows the connections of the external memory configuration, consisting of 16KB of EPROM and 8KB of static RAM.