Virtual 8086 Mode. More...

Collaboration diagram for VM86:

Data Structures
struct	vm86_farptr_t
	A real mode far pointer. More...

Macros
#define	CODE_ADDRESS ((void*) 0x500)
	Where the assembly code will be located. More...

#define	IVT_ADDRESS ((void*) 0)
	The real mode IVT (= Interrupt Vector Table) lies at the start of memory. More...

#define	OPERAND_SIZE 0x66
	opcode for overriding operand size

#define	OPCODE_PUSHF 0x9C
	PUSHF triggers a GPF inside VM86 mode.

#define	OPCODE_POPF 0x9D
	POPF triggers a GPF inside VM86 mode.

#define	OPCODE_INT_3 0xCC
	INT $3 triggers a GPF inside VM86 mode.

#define	OPCODE_INT 0xCD
	INT triggers a GPF inside VM86 mode.

#define	OPCODE_IRET 0xCF
	IRET triggers a GPF inside VM86 mode.

#define	OPCODE_CLI 0xFA
	CLI triggers a GPF inside VM86 mode.

#define	OPCODE_STI 0xFB
	STI triggers a GPF inside VM86 mode.

#define	CASE_IN(opcode, in_func, operand, inc, type)
	Emulates an IN instruction. More...

#define	CASE_OUT(opcode, out_func, operand, inc)
	Emulates an OUT instruction. More...

Functions
static vm86_farptr_t	vm86_get_farptr (void *addr)
	Translate an address into a far pointer. More...

static void	vm86_write_farptr (uint16_t segment, uint16_t offset, vm86_farptr_t farptr)
	Writes a far pointer into another location. More...

static void *	vm86_get_address (vm86_farptr_t farptr)
	Translate a far pointer into an address. More...

task_pid_t	vm86_create_task (void code_start, void code_end, page_directory_t page_directory, size_t kernel_stack_len, size_t user_stack_len, isr_registers_t registers)
	Creates a VM86 task. More...

void	vm86_call_bios (uint8_t interrupt, isr_registers_t *registers)
	Calls a BIOS interrupt. More...

static void	vm86_push (cpu_state_t *cpu, uint16_t value)
	Emulates a PUSH instruction. More...

static uint16_t	vm86_pop (cpu_state_t *cpu)
	Emulates a POP instruction. More...

static void	vm86_increment_eip (cpu_state_t *cpu, size_t inc)
	Emulates the completion of an instruction. More...

static uint8_t	vm86_monitor (cpu_state_t *cpu)
	Monitors a VM86 task by emulating sensitive instructions. More...

static cpu_state_t *	vm86_handle_gpf (cpu_state_t *cpu)
	Handles general protection faults. More...

void	vm86_init ()
	Initializes VM86 mode.

Variables
static vm86_farptr_t *	ivt = IVT_ADDRESS
	IVT needed to do BIOS calls.

const void	vm86_call_bios_start
	start of the BIOS call code

const void	vm86_call_bios_end
	end of the BIOS call code

const void	vm86_interrupt_hook
	location of INT in the BIOS call code

Detailed Description

Virtual 8086 Mode.

The VM86 mode runs 16-bit code and calls BIOS functions. A VM86 monitor is needed to emulate sensitive instructions.

See also: vm86_asm.S; http://wiki.osdev.org/Virtual_8086_Mode; http://www.lowlevel.eu/wiki/Virtual_8086_Mode; http://osdev.berlios.de/v86.html; http://www.rcollins.org/articles/pvi1/pvi1.html; http://lowlevel.eu/wiki/Speicherbereiche; http://www.brokenthorn.com/Resources/OSDevVid2.html; http://ref.x86asm.net/coder32.html

Macro Definition Documentation

#define CASE_IN	(	opcode,
		in_func,
		operand,
		inc,
		type
	)

Value:

case (opcode): \
        *(type*) &cpu->r.eax = (in_func)(operand); \
        vm86_increment_eip(cpu, (inc)); \
        break;

Emulates an IN instruction.

Parameters

opcode	the opcode of the instruction to emulate
in_func	the function to call
operand	the input port
inc	the instruction length in bytes
type	in_func's return type

Definition at line 54 of file vm86.c.

#define CASE_OUT	(	opcode,
		out_func,
		operand,
		inc
	)

Value:

case (opcode): \
        (out_func)((operand), cpu->r.eax); \
        vm86_increment_eip(cpu, (inc)); \
        break;

Emulates an OUT instruction.

Parameters

opcode	the opcode of the instruction to emulate
out_func	the function to call
operand	the output port
inc	the instruction length in bytes

Definition at line 67 of file vm86.c.

#define CODE_ADDRESS ((void*) 0x500)

Where the assembly code will be located.

The code will be memcpy'd to this location. We set this to the beginning of conventional memory. This way only one VM86 task can be run at a time!

Definition at line 33 of file vm86.c.

#define IVT_ADDRESS ((void*) 0)

The real mode IVT (= Interrupt Vector Table) lies at the start of memory.

One of the few occasions to dereference a null pointer ^^

Definition at line 36 of file vm86.c.

Function Documentation

void vm86_call_bios	(	uint8_t	interrupt,
		isr_registers_t *	registers
	)

Calls a BIOS interrupt.

Parameters

interrupt	the interrupt vector to call
registers	the parameters for the call

Definition at line 205 of file vm86.c.

Here is the call graph for this function:

task_pid_t vm86_create_task	(	void *	code_start,
		void *	code_end,
		page_directory_t *	page_directory,
		size_t	kernel_stack_len,
		size_t	user_stack_len,
		isr_registers_t *	registers
	)

Creates a VM86 task.

See also: task_create_detailed

Parameters

code_start	start of the 16-bit code
code_end	end of the 16-bit code
page_directory	a page directory for the task, if 0 a new one is created
kernel_stack_len	number of bytes to allocate for the kernel stack
user_stack_len	number of bytes to allocate for the user stack
registers	parameters to pass to the 16-bit code

Returns: the task's PID

Identity maps the first MiB so our VM86 task can operate inside it. Because this is not inside the user domain (1GiB and onwards), bypasses domain checking.

Copies the 16-bit code into conventional memory. We could map the code as virtual memory, but copying is easier ;) Note that this code might not reference any labels or so, because it needs to be position independent. For calling the BIOS this is enough, if we started writing real 16-bit programs, we would need to improve this.

The user stack is located after the code (we assume that's free memory).

Definition at line 145 of file vm86.c.

Here is the call graph for this function:

Here is the caller graph for this function:

static void* vm86_get_address ( vm86_farptr_t farptr )

static

Translate a far pointer into an address.

Parameters

farptr the far pointer

Returns: the address

Definition at line 129 of file vm86.c.

Here is the caller graph for this function:

static vm86_farptr_t vm86_get_farptr ( void * addr )

static

Translate an address into a far pointer.

Parameters

addr	the address

Returns: the far pointer

There are many ways to translate an address into a far pointer. We do it in such a way that our code is likely to only use one segment (which is easier than handling multiple segments). The equation used is:

1 paddr = segment * 16 + offset

Definition at line 91 of file vm86.c.

Here is the caller graph for this function: