diff --git a/arch/x64/boot.S b/arch/x64/boot.S
index f71960b094..bfca8c796b 100644
--- a/arch/x64/boot.S
+++ b/arch/x64/boot.S
@@ -67,6 +67,7 @@ interrupt_stack_top = .
 
 .globl start32
 start32:
+    # boot16.S set %eax to ELF start address, we'll use it later
     mov %eax, %ebp
     lgdt gdt_desc
     mov $0x10, %eax
@@ -101,6 +102,7 @@ start64:
     xor %eax, %eax
     rep stosb
     mov %rbp, elf_header
+    # %ebx is set by boot16.S before running the loader
     mov %rbx, osv_multiboot_info
     lea init_stack_top, %rsp
     call premain