RockOS/kernel/syscall.S

.code32 
.global syscall_handler
.extern syscall
syscall_handler: 
    cli

    pushfl          
    pushl %cs

    movl 8(%esp), %eax      
    pushl %eax
    movl 4(%esp), %ebx      # Get CS
    movl %ebx, 8(%esp)      # Move CS over Old EIP
    movl 0(%esp), %ebx      # Get Return EIP
    movl %ebx, 4(%esp)      # Move Return EIP over CS
    addl $4, %esp           # Adjust stack pointer

    movw %ds, %ax
    pushl %eax

    pushal

    pushl %esp
    call switch_context
    movl %eax, %esp

    popal
    
    popl %eax
    movw %ax, %ds
    movw %ax, %es
    movw %ax, %fs
    movw %ax, %gs

    sti
    iret