Commented Global Descriptors Table (GDT)

Author: Retr0-code

boot.asm

@@ -48,7 +48,7 @@ section .text:
 %include "functions/io.asm"
 %include "functions/disk_mng.asm"
 
-        msg: db "Hello, There!\nPlease enter your message: ", 0x0 ; Printable string
+        msg: db "Hello, There!\nPlease enter your message: \0" ; Printable string
     
         buffer: times 4 db 0        ; Defines input buffer
 
@@ -62,26 +62,32 @@ GDT_start:
         dd 0x0
 
     GDT_code:
-        dw 0xffff
-        dw 0x0
-        db 0x0
+        dw 0xffff           ; Segment limit (size)
+        dw 0x0              ; 8 bit base
+        db 0x0              ; 4 bit base
         db 0b10011010
+        ; 1 - defines a segment 00 - defines highest privilege 1 - defines code segment
+        ; 1 - code segment 0 - disable execution from low privileged segments 1 - readable 0 - access by CPU
         db 0b11001111
+        ; 1 - size *= 4096 1 - enable 32 bit memory
         db 0x0
 
     GDT_data:
-        dw 0xffff
-        dw 0x0
-        db 0x0
+        dw 0xffff           ; Segment limit (size)
+        dw 0x0              ; 8 bit base
+        db 0x0              ; 4 bit base
         db 0b10010010
+        ; 1 - defines a segment 00 - defines highest privilege 1 - defines code segment
+        ; 0 - data segment 0 - expandable upwords segment 1 - writable
         db 0b11001111
+        ; 1 - size *= 4096 1 - enable 32 bit memory
         db 0x0
 
         GDT_end:
 
     GDT_descriptor:
-        dw GDT_end - GDT_start - 1
-        dd GDT_start
+        dw GDT_end - GDT_start - 1      ; Size of GDT
+        dd GDT_start                    ; Start of GDT
 
     times 510-($-$$) db 0       ; Fills binary with 0 to keep the offset of 512
     dw 0xAA55                   ; Magic Word

functions/disk_mng.asm

@@ -12,22 +12,22 @@ section .text:
         mov dh, 0x00        ; Head that read
 
         int 0x13            ; BIOS interupt to read data
-        jc disk_error       ; If carry is set raise an error
+        jc _disk_error       ; If carry is set raise an error
 
         pop dx              ; Recover DX from stack
         cmp al, dh          ; Check amount of readen sectors
-        jne sectors_error   ; If they are not equal raise an error
+        jne _sectors_error   ; If they are not equal raise an error
         popa                ; Recover registers
         ret                 ; Exit the function
 
 
-    disk_error:
-        push DISK_ERROR
+    _disk_error:
+        push disk_error_text
         call _printformat
 
-    sectors_error:
-        push SECTORS_ERROR
+    _sectors_error:
+        push sectors_error_text
         call _printformat
 
-    DISK_ERROR: db "Disk read error", 0
-    SECTORS_ERROR: db "Incorrect number of sectors read", 0
+    disk_error_text: db "Disk read error\0"
+    sectors_error_text: db "Incorrect number of sectors read\0",

functions/printf.asm

@@ -46,7 +46,7 @@ section .text:
         mov al, [bx]                ; Moves addresses value to AL
         
         cmp al, 0x6e                ; If character is 'n'
-        je _newline                 ; Call new line
+        call _newline                 ; Call new line
         cmp al, 0x30                ; If character is '0'
         je _done_printf             ; Exits if was \0