1. VGA System Services
How to use Linux’s ‘vm86()’ system-call to access the video ROM-BIOS functions

2. The SVGA firmware
VESA-compliant graphics systems provide built-in service-functions (in adapter ROM)
Services normally execute during ‘startup’ before processor enters ‘protected mode’
But cpu can still ‘emulate’ 8086 behavior after system has entered protected mode (although kernel privileges are required)
Linux provides the system-call: ‘vm86()’

3. 8086 At startup, the Pentium operates like 8086:
Physical memory is directly addressable
But memory addresses are only 20-bits
CPU builds address from a pair of values:
Segment-address (16-bits) in special register
Offset-address (16-bits) in register or memory
Formula: address = (segment<<4) + offset
Address-range: 220 = 1,048,576 bytes

4. 8086 “real-mode” addresses
16-bits
16-bits
“logical”
address
(software)
segment-address
offset-address +
x 16
physical
address
(hardware)
20-bit bus address

5. Effect in the Pentium 4 Gigabyte address-range
1 Megabyte address-range

6. “Protected” Mode
At startup, the Pentium essentially IS an 8086 processor (operates in “real mode”):
It addresses physical memory like an 8086
It operates without any privilege-restrictions
But after building essential data-structures the Pentium switches to “protected” mode and turns on “virtual” memory-addressing:
To supports the execution of multiple tasks
To impose restrictions on memory access

7. Pentium can ‘emulate’ 8086
Even after it enters “protected” mode, the Pentium can still ‘emulate’ 8086 behavior
This works by creating a ‘virtual 8086’ cpu represented by a special data-structure in memory and triggered by a special opcode
But a few 8086 instructions aren’t allowed (if they could perhaps interfere with other tasks):
Device i/o: IN and OUT
Interrupts: CLI / STI, PUSHF / POPF, INT-n / IRET
Execution: HLT

8. Entering ‘virtual-8086’ mode= 1 N T =
-- GS
-- FS
-- DS
EFLAGS register-image
-- ES
-- SS
-- SP
EFLAGS
-- CS
SS:ESP
-- IP
Kernel’s instruction-stream
CS:EIP
iret
Kernel’s stack

9. Leaving “virtual-8086” mode
Once Pentium enters virtual-8086 mode, it leaves only when an interrupt or exception occurs (interrupts are caused by electrical signals from external devices, such as the keyboard or mouse -- or by the timer, and exceptions are caused by any attempts to execute “privileged” instructions, to violate the system’s protection restrictions, or to perform some kind of “illegal” operation

10. Handling exceptions
If an exception occurs while the Pentium is executing in ‘virtual-8086’ mode, registers are saved on the kernel stack and a kernel “exception-handler” is executed
The exception-handler might decide to go ahead and perform an operation (such as device i/o) that the ‘virtual-8086’ was not allowed to do on its own, and then resume executing the suspended virtual-8086 task

11. Linux’s ‘vm86()’ system-call
Linux allows (privileged) user-programs to invoke the Pentium’s capability to execute real-mode 8086 code in virtual-8086 mode
The user-program “submits” the required data-structure to the kernel, and the kernel enters ‘virtual-8086’ mode
If any restricted instruction is encountered, the kernel returns to the user-program the data-structure storing the saved task-state

12. Recall the LRMI
We used a software package called LRMI to assist us in executing ‘real mode’ code
The ‘mode3’ utility is built on this package
Now we shall see how LRMI really works!
We propose to write a ‘standalone’ demo- program that executes a useful ‘real mode’ video ROM-BIOS routine (using ‘vm86()’)

13. Linux Device-Drivers
We will need a way to ‘map’ certain special memory-regions into the user address-space
These regions must be mapped to addresses that an 8086 processor could access (i.e., must be in bottom one-megabyte of virtual memory)
Linux normally “maps” nothing else there
We’ll need device-drivers to perform mappings: /dev/zero (This is a standard part of Linux)
/dev/dos (This is a ‘custom’ driver we built)

14. How ‘/dev/zero’ works
This device lets a user map some unused pages of physical memory into user-space
As the name ‘zero’ suggests, the memory that is provided is initialized to ‘all-zeros’
This region will be used for the real mode code’s stack-area and data-structures; it could also be loaded with executable code

15. How ‘/dev/dos’ works
This device lets a user map conventional areas of initialized system memory into a user’s virtual address-space (such as the real-mode Interrupt Vector Table and the ROM-BIOS Data Area); and also the VGA system firmware!
There’s a similar device (‘/dev/mem’) that is a standard part of Linux, but it requires ‘root’ privileges for writing; so we substitute our own device-driver to avoid that ‘hassle’.

16. The 8086 memory-map ROM-BIOS 0xF0000 – 0xFFFFF Standard parts of the
PC design that much
code does rely upon
VGA ROM
0xC0000 – 0xCFFFF
VRAM
0xA0000 – 0xBFFFF
one
megabyte
Real Mode
Stack Area
Data and Text
This arena’s location and size can be
adjusted to suit our particular purpose
BIOS DATA
0x00400 – 0x00502
Standard parts of the
PC design that much
code does rely upon
IVT
0x00000 – 0x003FF

17. System preparation
Your system needs a device-node for the ‘/dev/dos’ device special file (normally it’s created by a Linux System Administrator)
But you can use ‘sudo’ to do it, like this: $ sudo mknod /dev/dos c $ sudo chmod a+rwx /dev/dos

18. The actual program-code
Use header-file: #include <sys/vm86.h>
Declare object: struct vm86_struct vm;
Map in the necessary memory-regions
Initialize memory-areas as appropriate
Initialize register-images in ‘vm86_struct’
Call kernel: int result = vm86( &vm );
Emulate any input and output instructions

19. Specific SVGA ROM function
We show how to execute VESA function 0
‘Get VESA BIOS-Extensions Information’)
It fills in the values of a data-structure that gives information about our SVGA system
Name of that data-structure is ‘VbeInfoBlk’
Structure-size is 512 bytes, documented in VESA white paper: ‘vbe3.pdf’ (on website)

20. The calling convention
VESA functions are designed to be called from an 8086 assembly-language program that is executing in real mode (i.e., startup)
Register AX is loaded with value 0x0F00
Registers ES:DI are loaded with address (segment:offset) of the memory-block that is to be filled in (512 bytes), initialized with 4-character string “VBE2”
Then software interrupt 0x10 is executed

21. Software interrupt instruction
The effect of executing ‘int-0x10’ is to transfer control to a function in ROM
The entry-point to that function was stored in the (real-mode) Interrupt Vector Table by the ROM-BIOS startup code
We can extract that entry-point address and use its pair of values as initial values for registers CS and IP in our virtual 8086

22. Emulating ‘in’ and ‘out’
Whenever an ‘in’ or ‘out’ instruction is encountered in ‘virtual-8086’ mode, our application can do an ‘emulation’
We decode the instruction and execute it on behalf of the virtual-8086 task, and we increment the IP value to “skip” past that ‘in’ or ‘out’ instruction, then we can resume the interrupted virtual-8086 task by calling ‘vm86()’ again, using the adjusted ‘vm’

23. Stopping the vm86 execution
We need a way to stop our execution-loop
We do it using a special 8086 instruction that cannot execute in virtual-8086 mode
Instead of emulating it, we stop our loop
Various instructions could serve this aim
We choose to use the ‘hlt’ instruction
It’s just 1-byte; its name suggests the idea

24. Demo-program: ‘vesainfo.cpp’
We’ve posted source-code for this demo
It implements the ideas we just discussed
It prints out the information in ‘VbeInfoBlk’
It is based on the VESA documentation
Exercise: You could try modifying our code to perform VESA function 1 (which is quite similar to function 0): it returns information about support for specific display-modes

25. Another application?
You can add some code to each case in the ‘my_emulate()’ function that prints out a description of a input or output operation
EXAMPLE: case 0xEE: // ‘outb’ opcode int al = vm->regs.eax & 0xFF; int dx = vm->regs.edx & 0xFFFF; printf( “outb( %02X, %04X )\n”, al, dx );
May help us “reverse engineer” VGA BIOS