/*
 * context_switch.S – cooperative context switch for x86_64
 *
 * void context_switch(UINT64 *old_rsp, UINT64 new_rsp);
 *
 *   %rdi = pointer to save current RSP  (old task's PCB field)
 *   %rsi = RSP value to restore         (new task's saved RSP)
 *
 * Saves callee-saved registers and RFLAGS on the current stack,
 * stores RSP, loads the new stack, restores registers, and returns
 * into the new task's execution context.
 */

.text
.global context_switch

context_switch:
    /* Save callee-saved registers and flags on the current stack */
    pushq   %rbp
    pushq   %rbx
    pushq   %r12
    pushq   %r13
    pushq   %r14
    pushq   %r15
    pushfq

    /* Save current stack pointer into *old_rsp */
    movq    %rsp, (%rdi)

    /* Load new task's stack pointer */
    movq    %rsi, %rsp

    /* Restore callee-saved registers and flags from the new stack */
    popfq
    popq    %r15
    popq    %r14
    popq    %r13
    popq    %r12
    popq    %rbx
    popq    %rbp

    ret

.section .note.GNU-stack,"",@progbits