/*
 * 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 (rbp, rbx, r12-r15) and RFLAGS on
 * the current stack, stores RSP into *old_rsp, loads new_rsp into
 * RSP, restores registers, and returns into the new task's context.
 *
 * This is a standard cooperative switch: only callee-saved state
 * needs preserving because the C calling convention guarantees that
 * caller-saved registers are already handled by the compiler.
 */

.text
.global context_switch

/* ----------------------------------------------------------------
 *  Entry: save old context, load new context, return
 * ---------------------------------------------------------------- */

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

    /* Store current RSP into *old_rsp */
    movq    %rsp, (%rdi)

    /* Switch to the new task's stack */
    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