Robert Clausecker <fuz(a)fuz.su> wrote: Technically speaking lcall $0x07,$0 uses selector 0 with RPL=3 (bit0 and bit1==1) and LDT (bit2==1) It seems it's oldest way to call kernel from userspace on x86 architecture. AT&T's programmers used this sycall convention for SysVR3 and SysVR4 on i386 (not sure about SysVR2 on i286). There are very few examples with lcall-type syscall i.e. http://www.sco.com/developers/devspecs/abi386-4.pdf (figure 3-26) (and leaked SysVR4 i386 sources) William Jolitz used this convention in his amazing articles about porting BSD4.3 to the i386 (c)1991 http://www.informatica.co.cr/unix-source-code/research/1991/0101.html (p."System Call Inteface"). See also 386BSD 0.0: https://github.com/386bsd/386bsd/blob/0.0/arch/i386/i386/locore.s#L361 (Did he run AT&T userspace on his kernel ???) As you mentioned, most of early *BSD systems on i386 also used lcall. Linus selected to use "DOS-style" call with INT 0x80. More recent BSD on i386 also use INT. https://john-millikin.com/unix-syscalls http://asm.sourceforge.net/intro/hello.html Solaris on x86 (ex. SysVR4) also uses lcall. See a https://www.cs.dartmouth.edu/sergey/cs258/solaris-on-x86.pdf p.4.2.3 and Solaris (later OpenSolaris and later Illumos) sourcecode.