LJMP

LJMP is an assembly language instruction used in the 8051 family of microcontrollers to perform a long, unconditional jump to a specified address in code memory. It enables program control to move anywhere within the 64K code space, outside the limitations of shorter jump forms.

Overview

LJMP transfers execution to a 16-bit address, allowing control flow to cross memory pages and reach locations that are not accessible with shorter jump instructions. It is a fundamental tool for implementing large programs or switching to routines located elsewhere in memory.

Syntax and encoding

In 8051 assembly, the form is typically written as LJMP addr16, where addr16 is a 16-bit target address. In machine code, LJMP is encoded as a three-byte instruction: one opcode byte followed by two bytes that specify the high and low parts of the destination address.

Comparison with other jumps

LJMP differs from shorter jumps such as AJMP, which uses a more limited addressing scheme within the current page. LJMP can reach any address in the 64K code space, making it suitable for jumps across pages or to external code areas when available.

Usage considerations

LJMP is unconditional and does not save a return address, so it is not appropriate for conventional subroutine calls. For subroutines, the ACALL instruction is preferred because it pushes a return address onto the stack. Because LJMP encodes a full 16-bit address, it can increase code size (three bytes) compared with shorter jumps. Its use is common in large programs or in startup and interrupt-handling routines where a jump to a distant location is required.

See also: AJMP, ACALL, RET.