51 MCU unconditional transfer instructions (4)

Control transfer instructions are used to control the flow of the program, and the controlled range is the program memory range. The MCS-51 series of microcontrollers have relatively rich control transfer instructions , including long call and long transfer instructions that can access 64kB program space address units, absolute call and absolute transfer instructions that can access 2kB bytes, short relative transfer within a page range and other unconditional transfer instructions. The execution of these instructions generally does not affect the flag bits.

[1]. Unconditional transfer instructions (4)

After this set of instructions is executed, the program will unconditionally transfer to the address pointed to by the instruction. The program memory space accessed by the long transfer instruction is 16 addresses and 64kB, and the program memory space accessed by the absolute transfer instruction is 11-bit address and 2kB space.

LJMP addr16; addr16 → (PC), assign a new value to the program counter (16-bit address)

AJMP addr11; (PC) + 2 → (PC), addr11 → (PC10-0) The program counter is assigned a new value (11-bit address), (PC15-11) is unchanged

SJMP rel; (PC) + 2 + rel → (PC) The current program counter is first added with 2 and then the offset to assign a new value to the program counter 

JMP @A+DPTR; (A) + (DPTR) → (PC), the value of the address unit pointed to by the accumulator plus the value of the data pointer assigns a new value to the program counter

If you want to analyze these instructions in detail, there are big differences. But for beginners, you can ignore so much and understand them all as LJMP labels, that is, jump to a label. But in fact, we have already come into contact with JMP labels in the previous routines, and we also know how to use them. The same is true for AJMP and SJMP. So what is the difference between these instructions? It lies in the different jump ranges. For example, for long jump, LJMP can jump 64K at a time (of course, it doesn't matter if it's closer). AJMP can only jump 2K at most, and SJMP can only jump 256 at most. In principle, all places where AJMP or SJMP is used can be replaced by LJMP. Therefore, for beginners, LJMP can be used when jumping.

However, be careful when looking up the table, as errors may occur because their machine cycles are different and the numbers obtained are also different.