Q.: What is the meaning of the line such as Init="b8024fbb1441cd10b8014fb91401cd10268b4d28268b552a" which can be seen along with other configuration strings in the early stage of booting?
A.: Really not so daunting. The string is machine code which is interpreted when the processor is in real mode before switching to protected mode, V86 mode or long mode. This is a simple and direct means to configure the display by setting up registers, writing to the low-memory setup area, and doing BIOS calls.
When the system is installed with Oberon0, an Init string is provided by Config.Mod. Then whenever the system is started, the boot loader allows the user to change the string.
Begin with the simple case, Init="b81200cd10". Here are three methods to disassemble the Init string.
1) You can use this web-based disassembler online: https://www.onlinedisassembler.com/odaweb/. For "Arch" choose "i8086" (the processor is in real mode) and paste the hex bytes of the Init string into the window below the platform setting. On the right hand side find the disassembled code.
.data:00000000 b81200 mov $0x12,%ax .data:00000003 cd10 int $0x10
2) We want a file Init.o containing these bytes. An obvious if not efficient method is to use Edit or ET to put any 5 characters in Init.o. Then Hex.Open Init.o, replace the characters with B8 12 00 CD 10 and Store. Move Init.o to a Linux system. There apply objdump.
objdump -b binary -m i8086 -D Init.o Init.o: file format binary Disassembly of section .data: 0000000000000000 <.data>: 0: b8 12 00 mov $0x12,%ax 3: cd 10 int $0x10
3) Kees C. suggests the libdisasm x86dis tool.
$ echo -e '\xb8\x12\x00\xcd\x10' | x86dis -r 0 5 -s att 00000000 B8 12 00 mov $0x0012, %ax 00000003 CD 10 int $0x10
Each method can be applied to any of the strings.
|b81200||mov||$0x12,%ax||Put 12h into register ax|
|cd10||int||$0x10||Given the content of ax, the video BIOS interrupt call will set 640 x 480 pixels x 24 colors.|
|b8024f||mov||$0x4f02,%ax||Put 4f02H in register ax.|
|bb1441||mov||$0x4114,%bx||Put 4114H in register bx.|
|cd10||int||$0x10||For these values in ax and bx, the video BIOS will set 800 x 600 pixels x ?? colors.|
|b8014f||mov||$0x4f01,%ax||Put 4f01H in ax.|
|b91401||mov||$0x114,%cx||Put 114H in cx to indicate 800 x 600 x 16 bit color.|
|cd10||int||$0x10||Video mode is queried. and es:di contains the returned information.|
|268b4d28||mov||%es:0x28(%di),%cx||rWinBAttrs = 0 or 1 according to window A or B supported.|
|268b552a||mov||%es:0x2a(%di),%dx||rWinBAttrs[a] = 1 or 0 according to window is writable or not.|
References[edit | edit source]
- Values are listed in https://pdos.csail.mit.edu/6.828/2008/readings/hardware/vgadoc/VGABIOS.TXT.
- This resolution is used in the Oberon0 installer. A higher resolution may be configured in the installed system.
- See http://www.techhelpmanual.com/938-int_10h_4f01h___query_supervga_mode_information.html
- See http://www.techhelpmanual.com/86-supervga_mode_info_block.html