boot/x86/stage3/loader.s

   1 bits 16
   2 cpu 686
   3 org 0x1800
   4
   5 %include "../fat32/fat32-structs.s"
   6
   7 %macro print 1
   8         push si
   9         mov si, %1
  10         call print_str
  11         pop si
  12 %endmacro
  13
  14 _start:
  15         mov [BOOT_DRIVE], dl
  16         call enable_unreal
  17         call check_a20
  18         jc .a20_enabled
  19         call enable_a20
  20         jnc .a20_enable_fail
  21 .a20_enabled:
  22         call get_1st_data_sec
  23         mov ax, 0x1000
  24         mov es, ax
  25         mov eax, BPB_RootClus
  26         xor di, di
  27         call read_cluster_chain
  28         jc critical_error
  29
  30         push cx
  31         push si
  32 .find_kernel:
  33         mov si, kernel_name
  34         cmp byte [es:di], 0 ; end of root directory
  35         je .kernel_missing
  36         cmp byte [es:di], 0xe5 ; unused entry
  37         je .increment
  38         mov cx, 11
  39         push si
  40         push di
  41         repe cmpsb
  42         pop di
  43         pop si
  44         je .kernel_found
  45 .increment:
  46         add di, 32
  47         jno .find_kernel
  48         mov bx, es
  49         add bx, 0x1000
  50         mov es, bx
  51         jmp .find_kernel
  52 .kernel_found:
  53         pop si
  54         pop cx
  55         mov ax, [es:di+dir_entry.firstclushi]
  56         shl eax, 16
  57         mov ax, [es:di+(dir_entry.firstcluslo)]
  58         mov edi, 0x100000
  59         call read_clus_chain_unreal ; load kernel
  60
  61         cli
  62         lgdt [gdt]
  63         mov eax, cr0
  64         or al, 1
  65         mov cr0, eax
  66         jmp 0x8:in_protected
  67 .kernel_missing:
  68         print missing_kernel
  69         jmp .halt
  70
  71 .a20_enable_fail:
  72         print a20_fail
  73 .halt:
  74         hlt
  75         jmp $-1
  76
  77 ; eax - start cluster
  78 ; edi - address to load to
  79 read_clus_chain_unreal:
  80         push eax
  81         push edi
  82 .loop:
  83         push di
  84         push es
  85         push ax
  86         xor di, di
  87         mov ax, 0x1000
  88         mov es, ax
  89         pop ax
  90         call read_cluster
  91         pop es
  92         pop di
  93         jc .done
  94         push esi
  95         push eax
  96         push ebx
  97         push ecx
  98         mov esi, 0x10000
  99         xor ebx, ebx
 100         movzx eax, word BPB_BytsPerSec
 101         movzx bx, byte BPB_SecPerClus
 102         mul ebx
 103         mov ecx, eax
 104         call memcpy
 105         add edi, ecx
 106         pop ecx
 107         pop ebx
 108         pop eax
 109         pop esi
 110         cmp eax, 0xffffff7
 111         jl .loop
 112 .done:
 113         pop edi
 114         pop eax
 115         ret
 116
 117 ; esi - copy from
 118 ; edi - copy to
 119 ; ecx - bytes to copy
 120 memcpy:
 121         push esi
 122         push edi
 123         push ecx
 124         push dx
 125
 126 .loop:
 127         mov dl, [esi]
 128         mov [edi], dl
 129         dec ecx
 130         cmp ecx, 0
 131         je .done
 132         inc esi
 133         inc edi
 134         jmp .loop
 135 .done:
 136         pop dx
 137         pop ecx
 138         pop edi
 139         pop esi
 140         ret
 141
 142
 143 %include "unreal.s"
 144 %include "a20.s"
 145 %include "../fat32/fat32.s"
 146 %include "gdt.s"
 147 %include "print.s"
 148
 149 in_protected:
 150 bits 32
 151         mov ax, 0x10
 152         mov ds, ax
 153         mov es, ax
 154         mov ss, ax
 155         mov fs, ax
 156         mov gs, ax
 157         call 0x100000
 158         hlt
 159         jmp $-1
 160
 161 kernel_name: db "KERNEL  BIN"
 162 a20_fail: db "Failed to enable A20, giving up!", 0xd, 0xa, 0
 163 crit_err: db "A critical error occurred, dumping registers now: ", 0xd, 0xa, 0
 164 missing_kernel: db "Could not find KERNEL.BIN", 0xd, 0xa, 0
 165 eax_s: db "EAX: ", 0
 166 ebx_s: db "EBX: ", 0
 167 ecx_s: db "ECX: ", 0
 168 edx_s: db "EDX: ", 0
 169 esi_s: db "ESI: ", 0
 170 edi_s: db "EDI: ", 0
 171 cs_s: db "CS: ", 0
 172 ds_s: db "DS: ", 0
 173 es_s: db "ES: ", 0
 174 ss_s: db "SS: ", 0
 175 space: db " ", 0
 176 hex_delm: db "0x", 0
 177 newline: db 0xd, 0xa, 0