51 MCU memory

51 single chip microcomputer structure:

Thinking structure is important:

As can be seen from the figure above, the memory of the 51 microcontroller is divided into two parts: program memory ROM and data memory RAM, which is different from the computer

Program Memory ROM:

1. Program memory ROM, as the name implies, is where programs are stored. Program instructions guide the microcontroller to complete the set functions.

2. The 51 MCU has a 16-bit PC to indicate the location of the program instruction that the CPU will execute next time in the ROM. Since the PC pointer is 16 bits long, the size of the MCU's program memory ROM is 2^16 = 64Kb, from 0000 to FFFFH.

3. ROM can also be divided into on-chip ROM and off-chip ROM

On-chip ROM:

The 8051 chip has 4KB of ROM, with an address range from 0000H to 0FFFH.

Off-chip ROM:

Externally expandable, generally from 1000H to FFFFH, 64KB

PS: So the question is, how does the 51 microcontroller distinguish between internal ROM and external ROM?

The distinction between on-chip ROM and off-chip ROM:

When the microcontroller has both on-chip ROM and off-chip ROM, some duplicate address ranges will be generated.

In order to solve the problem of distinguishing between on-chip and off-chip ROM, the 51 microcontroller sets a control line EA (low level is valid), so

EA = 0 => valid => access to on-chip memory

EA = 1 => Invalid => On-chip memory is ignored

The essential principle:

That is, when the value of PC is greater than a certain value, the external memory will be accessed, and when the value of PC is less than a certain value, the internal memory will be accessed. This value is determined by the size of the memory capacity. In the 8051 microcontroller, the on-chip ROM = 4KB, so when PC is less than 0FFFH, the on-chip ROM is accessed.

Specific implementation process:

When EA = 0, access the external ROM directly from 0000H without hesitation

When EA = 1, PC starts accessing the on-chip ROM from 0000H. When PC is greater than 0FFFH, it switches to accessing the off-chip ROM, as shown in the figure:

There are 7 special addresses in the program memory ROM:

That is, the entry addresses of 7 external interrupts, each address is separated by 8 address units, which is obviously not enough to store the terminal service program, so jump instructions are generally stored in these 7 special addresses to jump to the corresponding interrupt service program.

Data Memory RAM:

1. RAM memory generally stores data needed during the operation of the microcontroller and temporarily generated data, and needs to be able to read and write quickly. It will be lost when power is off.

2. Like ROM, RAM is also divided into on-chip RAM and off-chip RAM

On-chip RAM:

1. 00H~1FH: working register area, also known as general register, 32 bytes, 8 bytes per group, total 4 groups.

In the special function register chapter, the RS0 and RS1 bits of the flag register PSW are used to select one of the four groups of registers to work, that is, one of the groups in this position is selected.

2. 20H~2FH: Bit addressing area, 16 bytes, 128 bits. The 128 bits in this address space can be accessed bit by bit. Each bit has an address. The addressing space is 00H ~ 7FH, as shown in the figure:

In the 20H~2FH space, each bit address is reassigned an address 00H ~ 7FH, thereby achieving bit-by-bit access.

3. General RAM area: also known as user RAM area, 80 bytes, for 52 series, generally from 30H to FFH for user use, the unused address units in the first two areas can also be used as user units

4. Stack area and stack pointer

It is usually set after the 2FH unit, avoiding the working register area and the bit addressing area. The typical application is the sub-function call:

Off-chip RAM:

Similar to ROM, RAM can be expanded through an external bus to obtain a larger storage space. Since the external bus width is 16 bits, the maximum external expansion is 64KB, and the address range is 0000H~FFFFH