stevia/src/mbr/mbr.nasm

; Copyright (c) 2023 Elaina Claus
;
; Permission is hereby granted, free of charge, to any person obtaining a copy
; of this software and associated documentation files (the "Software"), to deal
; in the Software without restriction, including without limitation the rights
; to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
; copies of the Software, and to permit persons to whom the Software is
; furnished to do so, subject to the following conditions:
;
; The above copyright notice and this permission notice shall be included in all
; copies or substantial portions of the Software.
;
; THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
; IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
; FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
; AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
; LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
; OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
; SOFTWARE.

[BITS 16]
[ORG 0x7A00]
[CPU KATMAI]
jmp short init
nop

; ###############
;
; Headers/Includes/Definitions
;
; ###############

%define __STEVIA_MBR

%include "cdecl16.inc"
%include "entry.inc"
%include "config.inc"
%include "mem.inc"
%include "error_codes.inc"
%include "partition_table.inc"

; ###############
; End Section
; ###############

ALIGN 4, db 0x90
init:
    cli                         ; We do not want to be interrupted

    xor ax, ax                  ; 0 AX
    mov ds, ax                  ; Set segment registers to 0

    mov ss, ax                  ; Set Stack Segment to 0
    mov sp, EARLY_STACK_START   ; Setup stack
    mov bp, sp                  ; base ptr = stack ptr
    sub sp, 0x20                ; local varible space             

    mov ch, 0x01                    ; 256 WORDs in MBR (512 bytes), 0x0100 in cx
    mov si, 0x7C00                  ; Current MBR Address (loaded here by BIOS)
    mov di, MBR_ENTRY               ; New MBR Address (our new relocation address)
    rep movsw                       ; copy 512 bytes from 0x0000:7c00 to 0x0000:MBR_ENTRY (7A00 as of writing)

    sti

    jmp 0:main

; ###############
;
; Extra/Shared Functions
;
; ###############

%include "kmem_func.inc"
%include "util/error_func.inc"

; ###############
; End Section
; ###############

;
; bp - 2 : uint8_t boot_drive
; bp - 4 : uint16_t part_offset
;

ALIGN 4, db 0x90
main:
    mov byte [bp - 2], dl            ; BIOS passes drive number in DL

    .check_disk:
        cmp dl, 0x80
        jae main.check_extentions
        ERROR MBR_ERROR_DISK_T_ERR

    .check_extentions:
        xor ax, ax
        mov bx, 0x55AA
        mov dl, byte [bp - 2]
        int 0x13
        jnc main.find_active
        ERROR MBR_ERROR_NO_INT32E                         ; no extended function support

    .find_active:
        mov bx, PartEntry1                      ; base = first partition
        mov cx, 4                               ; there are only 4 entries
    .find_active_L0:
        mov al, byte [bx + PartEntry_t.attributes]
        test al, 0x80                           ; 0x80 == 1000_0000b
        jnz main.active_found

        add bx, 0x10                            ; add 16 bytes to offset
        loop main.find_active_L0
        ERROR MBR_ERROR_NO_NO_BOOT_PART

    .active_found:
        mov ax, bx
        sub ax, DiskSig                         ; leaves us with the offset relative from start of table
                                                ; this gives us an offset from the begining of the partition table
        mov word [bp - 4], ax                   ; update part_offset
    .read_data:
        movzx ax, byte [bp - 2]
        push ax                                            ; drive_num

        mov ax, 0x1
        push ax                                            ; count

        mov dword eax, dword [bx + PartEntry_t.lba_start]
        push dword eax                                     ; lba

        xor ax, ax
        push ax                                            ; offset = 0

        mov ax, VBR_ENTRY
        shr ax, 4
        push ax                                            ; segment = 7C0

        ; uint8_t read_stage2_raw(uint16_t buf_segment, uint16_t buf_offset, 
        ;                         uint32_t lba,
        ;                         uint16_t count, uint16_t drive_num)
        call read_disk_raw
        add sp, 0xC
    .goto_vbr:
        cmp word [VBR_ENTRY + 0x1FE], 0xAA55
        je main.sig_ok
        ERROR MBR_ERROR_NO_VBR_SIG                        ; no signature present
    .sig_ok:
        mov ax, partition_table_SIZE            ; 72 bytes of data
        push ax
        mov ax, DiskSig                         ; start of partition table
        push ax
        mov ax, partition_table                 ; defined in memory.inc
        push ax
        call kmemcpy                            ; copy partition table to memory
        add sp, 0x6

        mov si, word [bp - 4]
        mov dl, byte [bp - 2]
        jmp 0:0x7C00

; ###############
;
; BIOS Functions
;
; ###############

%include 'BIOS/func/ext_read.inc'

; ###############
; End Section
; ###############

%assign bytes_remaining (440 - ($ - $$))
%warning MBR has bytes_remaining bytes remaining for code (MAX: 440 bytes)
times ((512 - 72) - ($ - $$)) nop     ; Fill the rest of sector with nop

DiskSig:
    times 4 db 0x00
Reserved1:
    db 0x00
Reserved2:
    db 0x00

PartEntry1:
    times 16 db 0x00
PartEntry2:
    times 16 db 0x00
PartEntry3:
    times 16 db 0x00
PartEntry4:
    times 16 db 0x00
BootSig:
    dw 0xAA55                    ; Add boot signature at the end of bootloader