It is not clear if you are working with Keil C51 or A51. I will assume that you are woring in A51 with assember language labels.

I tested label length in Keil A51 using the compiler tool set version as shown:

Tool Version Numbers:
Toolchain Path:    C:KeilC51BIN
C Compiler:         C51.exe       V7.01
Assembler:          A51.exe       V7.01
Linker/Locator:     BL51.exe      V5.01
Librarian:          LIB51.exe     V4.24
Hex Converter:      OH51.exe      V2.6
Simulation DLL:     S8051.DLL     V2.31
Dialog DLL:         DP51.DLL      V2.31

I made up a program that contained the following code. I compiled and tested the code in the Keil simulator in a repeated manner by making the label lengths longer and longer until it did not work properly. Here the length is at 57 characters before the assembler accused me of trying to allocate duplicate labels.

main:
    nop
    nop
    nop
main56789012345678901234567890123456789012345678901234567:
    call    rand8
main56789012345678901234567890123456789012345678901234567t:
    jmp    main56789012345678901234567890123456789012345678901234567

You should be able to perform a very similar test in your version of Keil A51 to determine the maximum label length. It should also be possible to craft a very similar procedure in C51 to check the limits of the C compiler.

Michael Karas