; STAGE 2 org 0x8000 ; start address for the second stage bootloader , padding of 512 bytes from stage 1 ; CONSTANTS newline equ 0x0a carriageReturn equ 0x0d teletype_function equ 0x0e printInterrupt equ 0x10 ; biosSystemInterrupt equ 0x15 ; a20SupportStatus equ 0x2403 ; a20Gatestatus equ 0x2402 ; a20gateenable equ 0x2401W ; STACK AND INTERRUPTS start: ; setting up fresh segments for the stack cli xor ax, ax mov ds, ax mov es, ax mov ax, 0x9000 mov ss, ax ; ss can not be directly assigned a vlaue, do it via the ax register mov sp, 0x0000 ; stack pointer is set to the maximum it can go just before the reserved memory ; stack grows downwards sti mov si,stage2LoadMessage print_message_1: ; loop to print the message lodsb cmp al,0 je new_line mov ah, teletype_function int printInterrupt jmp print_message_1 new_line: ; to print the newline mov al,newline mov ah,teletype_function int printInterrupt mov al,carriageReturn mov ah,teletype_function int printInterrupt jmp enable_a20 ; https://wiki.osdev.org/A20_Line ; https://www.ctyme.com/intr/int-15.html ; jnz stands for jump if not zero ;enable_a20: ; mov ax, a20SupportStatus ; checking if bios supports a20 gate by default, placing the query function in the ax register ; int biosSystemInterrupt ; bios system interrupt to querry for the function ; jc a20_not_supported ; checkking for the value in the jump cary flag if the value is 1 show an error, if 0x15 is not supported it is set as 1 ; test ah, ah ; checking if the ah register is 0 or not through a logical AND; important to check as there are 3 responses a bios may submit ; jnz a20_not_supported ; if logical AND does not return a 0 throw an error, sets ZF=1 if AH is 0, jump if not zero = jump if AH != 0. ; mov ax, a20Gatestatus ; querying for the gate status for a20 ; int biosSystemInterrupt ; bios system interrupt to query for the function ; jc a20_not_enabled ; if the carry flag is set jump to the error ; test ah, ah ; if ah valur is 1 then its an error ; jnz a20_not_enabled ; if value is 1 jump to the error ; test al, al ;if al value is 1 then a20 is on if 0 then it is off ; jnz a20_activated ; if al is on jump to activated ; ; ah is the value for bios and al is the response ; mov ax, a20gateenable ; Activate A20 gate ; int biosSystemInterrupt ; bios system interrupt to query for the function ; jc a20_not_enabled ; if the carry flag is set then the activation failed and fo to the error message ; test ah, ah ; if ah is 1 then it failed ; jnz a20_not_enabled ; if failed go to the error messgae ; a20_not_supported: ; jump to infinite loop ; jmp stage2_loop ; a20_not_enabled: ; jump to infinite loop ; jmp stage2_loop ; a20_activated: ; a20 activated si, message, and loop ; mov si,a20ActivatedMessage ; a20_activated_print: ; lodsb ; cmp al,0 ; je stage2_loop ; mov ah, teletype_function ; int printInterrupt ; jmp a20_activated_print ; A20 using system control port A ; found on most newer computers ;https://wiki.osdev.org/A20_Line#Fast_A20_Gate enable_a20: in al, 0x92 ; in reads data from the hardware i/o port to a cpu register ; reading flom the port number 0x92 and storing it in the register al or al, 2 ; performing a bitwise operator with al and 2 ; this sets the second bit from the right 1 without changing the value of other bits out 0x92, al ; writes the value from al register to the hardware port ; so it is duet o ibm pc design language where keyboard port was used to enable a20 line by modify the value of ports around it an a widely used port was 0x92 jmp protected_mode_setup ; DEFINING THE GDT TABLE gdt_start: dq 0x0000000000000000 ; defining the value for a null segment dw 0xffff ; maximum limit of the segment 4 gb dw 0x0000 ; lower base value db 0x00 ; middle base value db 0x9a ; access right db 0xcf ; flag and the the upper limit limit db 0x00 ; higher base value ; so the base is 0x00000000 ; the lower limit for the segment is 0xffff ; the upper limit of the segment is 0xf ; the flag for this section is 1100 ; combining the flag and limit to a byte we get 0x1100f or 0x11001111 ; in acess byte 00: is kernel, 01(drivers) & 10(services) are rarely used, 11 is for userspace ; the acess byte is an hexadecimal value for thhe binary value 10011010 dw 0xffff dw 0x0000 db 0x00 db 0x92 db 0xcf db 0x00 ; everything is the same besifdes the acess byte its value is 10010010 gdt_end: ; TELLING THE CPU HOW LARGE THE GDT TABLE IS POINTER TO THE TABLE ; gdt descriptor assumes the size of the table is always size-1 gdt_descriptor: dw gdt_end - gdt_start - 1 ; size of gdt -1 dd gdt_start ; starting address if the gdt ; FAR JUMP TO THE PROTECTED MODE protected_mode_setup: cli lgdt [gdt_descriptor] ; load the gdt table mov eax, cr0 ; reading the value of the control register or eax, 1 ; setting the right most bit to one mov cr0, eax ; setting the value of the control register to the updated value ; cr0 is the cpus main mode control registerit contains flags about how the processor behaves ; watch https://www.youtube.com/watch?v=BQGbBCqUGCc ; right most bit setting it to 1 enables the protection a20_not_enabled ; tells the cpu that we are in protected mode now ; cr0 is not the ony control register there are more registers that do various jobs jmp 0x08:protected_mode ; jumping to the code segment in gdt and calling protected mode lable ; jumping to the code segment as it allows the execution of instructions ; without declaring bits 32 we get the following error, telling assembler generate for 32 bit instruction ; ./src/bootStageTwo.asm:157: warning: word data exceeds bounds [-w+number-overflow] ; ./src/bootStageTwo.asm:157: warning: word data exceeds bounds [-w+number-overflow] [bits 32] protected_mode: ; setting the stack for the protected mode mov ax, 0x10 ; setting the protected mode stack mov ds, ax mov ss, ax mov es, ax mov fs, ax mov gs, ax mov ebp, 0x9ffff ; setting the stack at 0x9ffff in flat memory model mov esp, ebp mov dword [0xb8000], 0x07200750 stage2_loop: ; infinite loop to stop the bootloader from crashing jmp $ mov dword [0xb8000], 0x07200750 ; MESSAGES stage2LoadMessage db "ENTERED STAGE 2",0 ; a20ActivatedMessage db "A20 ACTIVATED",0