Introduce a kernel_yield function to use once a scheduler is implemented

kernel/src/arch/CPU.h

@@ -19,3 +19,5 @@ namespace CPU
     void disable_interrupts();
     void wait_for_interrupt();
 }
+
+extern "C" void kernel_yield();

kernel/src/arch/x86_64/CPU.asm

@@ -48,6 +48,51 @@ load_tr:
     ltr ax
     ret
 
+global switch_task
+switch_task:
+    cli
+.loop:    
+    hlt
+    jmp .loop
+
+global kernel_yield
+kernel_yield:
+    mov rdi, [rsp] ; return address is now in RDI
+    mov rcx, rsp ; save current RSP
+    add rcx, 8 ; skip over the return address
+
+    mov eax, ss
+    push rax ; SS
+    push rcx ; RSP
+    pushfq ; RFLAGS
+    mov eax, cs
+    push rax ; CS
+    push rdi ; RIP
+
+    ; ISR and error code
+    push 0
+    push 0
+
+    push 0 ; RAX
+    push rbx ; Preserve RBX
+    push 0 ; RCX
+    push 0 ; RDX
+    push 0 ; RSI
+    push 0 ; RDI
+    push rbp ; Preserve RBP
+    push 0 ; R8
+    push 0 ; R9
+    push 0 ; R10
+    push 0 ; R11
+    push r12 ; Preserve R12
+    push r13 ; Preserve R13
+    push r14 ; Preserve R14
+    push r15 ; Preserve R15
+
+    mov rdi, rsp
+    call switch_task
+    jmp _asm_interrupt_exit
+
 %macro ISR 1
     global _isr%1
     _isr%1:
@@ -97,6 +142,7 @@ _asm_interrupt_entry:
     mov rdi, rsp
     call arch_interrupt_entry
 
+_asm_interrupt_exit:
     pop r15
     pop r14
     pop r13