Add more explanations to some opaque x86 instructions (#19)

src/boot.s

@@ -12,7 +12,8 @@ _start:
     mov fs, ax
     mov gs, ax
 
-    # TODO explain
+    # clear the direction flag (e.g. go forward in memory when using
+    # instructions like lodsb)
     cld
 
 
@@ -29,6 +30,7 @@ enable_a20:
     out 0x92, al
 
 enter_protected_mode:
+    # clear interrupts
     cli
     push ds
     push es
@@ -118,6 +120,9 @@ println:
 print:
     cld
 print_loop:
+    # note: if direction flag is set (via std)
+    # this will DECREMENT the ptr, effectively
+    # reading/printing in reverse.
     lodsb al, BYTE PTR [esi]
     test al, al
     jz print_done

src/second_stage.s

@@ -56,7 +56,11 @@ load_next_kernel_block_from_disk:
     push ecx
     push esi
     mov ecx, 512 / 4
+    # move with zero extension
+    # because we are moving a word ptr
+    # to esi, a 32-bit register.
     movzx esi, word ptr [dap_buffer_addr]
+    # move from esi to edi ecx times.
     rep movsd [edi], [esi]
     pop esi
     pop ecx
@@ -256,7 +260,7 @@ gdt_64_pointer:
 
 long_mode:
     # call load_elf with kernel start address, size, and memory map as arguments
-    movabs rdi, 0x400000
+    movabs rdi, 0x400000 # move absolute 64-bit to register
     mov rsi, _kib_kernel_size
     lea rdx, _memory_map
     movzx rcx, word ptr mmap_ent