Chapter 15    Virtual 8088 Mode

Chapter 15 Virtual 8088 Mode

Chapter 15  Virtual 8086 Mode


The 80386 supports execution of one or more 8086, 8088, 80186, or 80188
programs in an 80386 protected-mode environment. An 8086 program runs in
this environment as part of a V86 (virtual 8086) task. V86 tasks take
advantage of the hardware support of multitasking offered by the protected
mode. Not only can there be multiple V86 tasks, each one executing an 8086
program, but V86 tasks can be multiprogrammed with other 80386 tasks.

The purpose of a V86 task is to form a "virtual machine" with which to
execute an 8086 program. A complete virtual machine consists not only of
80386 hardware but also of systems software. Thus, the emulation of an 8086
is the result of cooperation between hardware and software:

  *  The hardware provides a virtual set of registers (via the TSS), a
     virtual memory space (the first megabyte of the linear address space of
     the task), and directly executes all instructions that deal with these
     registers and with this address space.

  *  The software controls the external interfaces of the virtual machine
     (I/O, interrupts, and exceptions) in a manner consistent with the
     larger environment in which it executes. In the case of I/O, software
     can choose either to emulate I/O instructions or to let the hardware
     execute them directly without software intervention.

Software that helps implement virtual 8086 machines is called a V86

15.1 Executing 8086 Code

15.1 Executing 8086 Code The processor executes in V86 mode when the VM (virtual machine) bit in the EFLAGS register is set. The processor tests this flag under two general conditions: 1. When loading segment registers to know whether to use 8086-style address formation. 2. When decoding instructions to determine which instructions are sensitive to IOPL. Except for these two modifications to its normal operations, the 80386 in V86 mode operated much as in protected mode.

15.1.1 Registers and Instructions

15.1.1 Registers and Instructions The register set available in V86 mode includes all the registers defined for the 8086 plus the new registers introduced by the 80386: FS, GS, debug registers, control registers, and test registers. New instructions that explicitly operate on the segment registers FS and GS are available, and the new segment-override prefixes can be used to cause instructions to utilize FS and GS for address calculations. Instructions can utilize 32-bit operands through the use of the operand size prefix. 8086 programs running as V86 tasks are able to take advantage of the new applications-oriented instructions added to the architecture by the introduction of the 80186/80188, 80286 and 80386: * New instructions introduced by 80186/80188 and 80286. -- PUSH immediate data -- Push all and pop all (PUSHA and POPA) -- Multiply immediate data -- Shift and rotate by immediate count -- String I/O -- ENTER and LEAVE -- BOUND * New instructions introduced by 80386. -- LSS, LFS, LGS instructions -- Long-displacement conditional jumps -- Single-bit instructions -- Bit scan -- Double-shift instructions -- Byte set on condition -- Move with sign/zero extension -- Generalized multiply

15.1.2 Linear Address Formation

15.1.2 Linear Address Formation In V86 mode, the 80386 processor does not interpret 8086 selectors by referring to descriptors; instead, it forms linear addresses as an 8086 would. It shifts the selector left by four bits to form a 20-bit base address. The effective address is extended with four high-order zeros and added to the base address to create a linear address as Figure 15-1 illustrates. Because of the possibility of a carry, the resulting linear address may contain up to 21 significant bits. An 8086 program may generate linear addresses anywhere in the range 0 to 10FFEFH (one megabyte plus approximately 64 Kbytes) of the task's linear address space. V86 tasks generate 32-bit linear addresses. While an 8086 program can only utilize the low-order 21 bits of a linear address, the linear address can be mapped via page tables to any 32-bit physical address. Unlike the 8086 and 80286, 32-bit effective addresses can be generated (via the address-size prefix); however, the value of a 32-bit address may not exceed 65,535 without causing an exception. For full compatibility with 80286 real-address mode, pseudo-protection faults (interrupt 12 or 13 with no error code) occur if an address is generated outside the range 0 through 65,535. See Also:

15.2 Structure of a V86 Task

15.2 Structure of a V86 Task A V86 task consists partly of the 8086 program to be executed and partly of 80386 "native mode" code that serves as the virtual-machine monitor. The task must be represented by an 80386 TSS (not an 80286 TSS). The processor enters V86 mode to execute the 8086 program and returns to protected mode to execute the monitor or other 80386 tasks. To run successfully in V86 mode, an existing 8086 program needs the following: * A V86 monitor. * Operating-system services. The V86 monitor is 80386 protected-mode code that executes at privilege-level zero. The monitor consists primarily of initialization and exception-handling procedures. As for any other 80386 program, executable-segment descriptors for the monitor must exist in the GDT or in the task's LDT. The linear addresses above 10FFEFH are available for the V86 monitor, the operating system, and other systems software. The monitor may also need data-segment descriptors so that it can examine the interrupt vector table or other parts of the 8086 program in the first megabyte of the address space. In general, there are two options for implementing the 8086 operating system: 1. The 8086 operating system may run as part of the 8086 code. This approach is desirable for any of the following reasons: * The 8086 applications code modifies the operating system. * There is not sufficient development time to reimplement the 8086 operating system as 80386 code. 2. The 8086 operating system may be implemented or emulated in the V86 monitor. This approach is desirable for any of the following reasons: * Operating system functions can be more easily coordinated among several V86 tasks. * The functions of the 8086 operating system can be easily emulated by calls to the 80386 operating system. Note that, regardless of the approach chosen for implementing the 8086 operating system, different V86 tasks may use different 8086 operating systems.

15.2.1 Using Paging for V86 Tasks

15.2.1 Using Paging for V86 Tasks Paging is not necessary for a single V86 task, but paging is useful or necessary for any of the following reasons: * To create multiple V86 tasks. Each task must map the lower megabyte of linear addresses to different physical locations. * To emulate the megabyte wrap. On members of the 8086 family, it is possible to specify addresses larger than one megabyte. For example, with a selector value of 0FFFFH and an offset of 0FFFFH, the effective address would be 10FFEFH (one megabyte + 65519). The 8086, which can form addresses only up to 20 bits long, truncates the high-order bit, thereby "wrapping" this address to 0FFEFH. The 80386, however, which can form addresses up to 32 bits long does not truncate such an address. If any 8086 programs depend on this addressing anomaly, the same effect can be achieved in a V86 task by mapping linear addresses between 100000H and 110000H and linear addresses between 0 and 10000H to the same physical addresses. * To create a virtual address space larger than the physical address space. * To share 8086 OS code or ROM code that is common to several 8086 programs that are executing simultaneously. * To redirect or trap references to memory-mapped I/O devices.

15.2.2 Protection within a V86 Task

15.2.2 Protection within a V86 Task Because it does not refer to descriptors while executing 8086 programs, the processor also does not utilize the protection mechanisms offered by descriptors. To protect the systems software that runs in a V86 task from the 8086 program, software designers may follow either of these approaches: * Reserve the first megabyte (plus 64 kilobytes) of each task's linear address space for the 8086 program. An 8086 task cannot generate addresses outside this range. * Use the U/S bit of page-table entries to protect the virtual-machine monitor and other systems software in each virtual 8086 task's space. When the processor is in V86 mode, CPL is 3. Therefore, an 8086 program has only user privileges. If the pages of the virtual-machine monitor have supervisor privilege, they cannot be accessed by the 8086 program.

15.3 Entering and Leaving V86 Mode

15.3 Entering and Leaving V86 Mode Figure 15-2 summarizes the ways that the processor can enter and leave an 8086 program. The processor can enter V86 by either of two means: 1. A task switch to an 80386 task loads the image of EFLAGS from the new TSS. The TSS of the new task must be an 80386 TSS, not an 80286 TSS, because the 80286 TSS does not store the high-order word of EFLAGS, which contains the VM flag. A value of one in the VM bit of the new EFLAGS indicates that the new task is executing 8086 instructions; therefore, while loading the segment registers from the TSS, the processor forms base addresses as the 8086 would. 2. An IRET from a procedure of an 80386 task loads the image of EFLAGS from the stack. A value of one in VM in this case indicates that the procedure to which control is being returned is an 8086 procedure. The CPL at the time the IRET is executed must be zero, else the processor does not change VM. The processor leaves V86 mode when an interrupt or exception occurs. There are two cases: 1. The interrupt or exception causes a task switch. A task switch from a V86 task to any other task loads EFLAGS from the TSS of the new task. If the new TSS is an 80386 TSS and the VM bit in the EFLAGS image is zero or if the new TSS is an 80286 TSS, then the processor clears the VM bit of EFLAGS, loads the segment registers from the new TSS using 80386-style address formation, and begins executing the instructions of the new task according to 80386 protected-mode semantics. 2. The interrupt or exception vectors to a privilege-level zero procedure. The processor stores the current setting of EFLAGS on the stack, then clears the VM bit. The interrupt or exception handler, therefore, executes as "native" 80386 protected-mode code. If an interrupt or exception vectors to a conforming segment or to a privilege level other than three, the processor causes a general-protection exception; the error code is the selector of the executable segment to which transfer was attempted. Systems software does not manipulate the VM flag directly, but rather manipulates the image of the EFLAGS register that is stored on the stack or in the TSS. The V86 monitor sets the VM flag in the EFLAGS image on the stack or in the TSS when first creating a V86 task. Exception and interrupt handlers can examine the VM flag on the stack. If the interrupted procedure was executing in V86 mode, the handler may need to invoke the V86 monitor. See Also:

15.3.1 Transitions Through Task Switches

15.3.1 Transitions Through Task Switches A task switch to or from a V86 task may be due to any of three causes: 1. An interrupt that vectors to a task gate. 2. An action of the scheduler of the 80386 operating system. 3. An IRET when the NT flag is set. In any of these cases, the processor changes the VM bit in EFLAGS according to the image of EFLAGS in the new TSS. If the new TSS is an 80286 TSS, the high-order word of EFLAGS is not in the TSS; the processor clears VM in this case. The processor updates VM prior to loading the segment registers from the images in the new TSS. The new setting of VM determines whether the processor interprets the new segment-register images as 8086 selectors or 80386/80286 selectors.

15.3.2 Transitions Through Trap Gates and Interrupt Gates

15.3.2 Transitions Through Trap Gates and Interrupt Gates The processor leaves V86 mode as the result of an exception or interrupt that vectors via a trap or interrupt gate to a privilege-level zero procedure. The exception or interrupt handler returns to the 8086 code by executing an IRET. Because it was designed for execution by an 8086 processor, an 8086 program in a V86 task will have an 8086-style interrupt table starting at linear address zero. However, the 80386 does not use this table directly. For all exceptions and interrupts that occur in V86 mode, the processor vectors through the IDT. The IDT entry for an interrupt or exception that occurs in a V86 task must contain either: * A task gate. * An 80386 trap gate (type 14) or an 80386 interrupt gate (type 15), which must point to a nonconforming, privilege-level zero, code segment. Interrupts and exceptions that have 80386 trap or interrupt gates in the IDT vector to the appropriate handler procedure at privilege-level zero. The contents of all the 8086 segment registers are stored on the PL 0 stack. Figure 15-3 shows the format of the PL 0 stack after an exception or interrupt that occurs while a V86 task is executing an 8086 program. After the processor stores all the 8086 segment registers on the PL 0 stack, it loads all the segment registers with zeros before starting to execute the handler procedure. This permits the interrupt handler to safely save and restore the DS, ES, FS, and GS registers as 80386 selectors. Interrupt handlers that may be invoked in the context of either a regular task or a V86 task, can use the same prolog and epilog code for register saving regardless of the kind of task. Restoring zeros to these registers before execution of the IRET does not cause a trap in the interrupt handler. Interrupt procedures that expect values in the segment registers or that return values via segment registers have to use the register images stored on the PL 0 stack. Interrupt handlers that need to know whether the interrupt occurred in V86 mode can examine the VM bit in the stored EFLAGS image. An interrupt handler passes control to the V86 monitor if the VM bit is set in the EFLAGS image stored on the stack and the interrupt or exception is one that the monitor needs to handle. The V86 monitor may either: * Handle the interrupt completely within the V86 monitor. * Invoke the 8086 program's interrupt handler. Reflecting an interrupt or exception back to the 8086 code involves the following steps: 1. Refer to the 8086 interrupt vector to locate the appropriate handler procedure. 2. Store the state of the 8086 program on the privilege-level three stack. 3. Change the return link on the privilege-level zero stack to point to the privilege-level three handler procedure. 4. Execute an IRET so as to pass control to the handler. 5. When the IRET by the privilege-level three handler again traps to the V86 monitor, restore the return link on the privilege-level zero stack to point to the originally interrupted, privilege-level three procedure. 6. Execute an IRET so as to pass control back to the interrupted procedure. See Also:

15.4 Additional Sensitive Instructions

15.4 Additional Sensitive Instructions When the 80386 is executing in V86 mode, the instructions PUSHF, POPF, INT n, and IRET are sensitive to IOPL. The instructions IN, INS, OUT, and OUTS, which are ordinarily sensitive in protected mode, are not sensitive in V86 mode. Following is a complete list of instructions that are sensitive in V86 mode: CLI -- Clear Interrupt-Enable Flag STI -- Set Interrupt-Enable Flag LOCK -- Assert Bus-Lock Signal PUSHF -- Push Flags POPF -- Pop Flags INT n -- Software Interrupt RET -- Interrupt Return CPL is always three in V86 mode; therefore, if IOPL < 3, these instructions will trigger a general-protection exceptions. These instructions are made sensitive so that their functions can be simulated by the V86 monitor.

15.4.1 Emulating 8086 Operating System Calls

15.4.1 Emulating 8086 Operating System Calls INT n is sensitive so that the V86 monitor can intercept calls to the 8086 OS. Many 8086 operating systems are called by pushing parameters onto the stack, then executing an INT n instruction. If IOPL < 3, INT n instructions will be intercepted by the V86 monitor. The V86 monitor can then emulate the function of the 8086 operating system or reflect the interrupt back to the 8086 operating system in V86 mode.

15.4.2 Virtualizing the Interrupt-Enable Flag

15.4.2 Virtualizing the Interrupt-Enable Flag When the processor is executing 8086 code in a V86 task, the instructions PUSHF, POPF, and IRET are sensitive to IOPL so that the V86 monitor can control changes to the interrupt-enable flag (IF). Other instructions that affect IF (STI and CLI) are IOPL sensitive both in 8086 code and in 80386/80386 code. Many 8086 programs that were designed to execute on single-task systems set and clear IF to control interrupts. However, when these same programs are executed in a multitasking environment, such control of IF can be disruptive. If IOPL is less than three, all instructions that change or interrogate IF will trap to the V86 monitor. The V86 monitor can then control IF in a manner that both suits the needs of the larger environment and is transparent to the 8086 program.

15.5 Virtual I/O

15.5 Virtual I/O Many 8086 programs that were designed to execute on single-task systems use I/O devices directly. However, when these same programs are executed in a multitasking environment, such use of devices can be disruptive. The 80386 provides sufficient flexibility to control I/O in a manner that both suits the needs of the new environment and is transparent to the 8086 program. Designers may take any of several possible approaches to controlling I/O: * Implement or emulate the 8086 operating system as an 80386 program and require the 8086 application to do I/O via software interrupts to the operating system, trapping all attempts to do I/O directly. * Let the 8086 program take complete control of all I/O. * Selectively trap and emulate references that a task makes to specific I/O ports. * Trap or redirect references to memory-mapped I/O addresses. The method of controlling I/O depends upon whether I/O ports are I/O mapped or memory mapped.

15.5.1 I/O-Mapped I/O

15.5.1 I/O-Mapped I/O I/O-mapped I/O in V86 mode differs from protected mode only in that the protection mechanism does not consult IOPL when executing the I/O instructions IN, INS, OUT, OUTS. Only the I/O permission bit map controls the right for V86 tasks to execute these I/O instructions. The I/O permission map traps I/O instructions selectively depending on the I/O addresses to which they refer. The I/O permission bit map of each V86 task determines which I/O addresses are trapped for that task. Because each task may have a different I/O permission bit map, the addresses trapped for one task may be different from those trapped for others. Refer to Chapter 8 for more information about the I/O permission map.

15.5.2 Memory-Mapped I/O

15.5.2 Memory-Mapped I/O In hardware designs that utilize memory-mapped I/O, the paging facilities of the 80386 can be used to trap or redirect I/O operations. Each task that executes memory-mapped I/O must have a page (or pages) for the memory-mapped address space. The V86 monitor may control memory-mapped I/O by any of these means: * Assign the memory-mapped page to appropriate physical addresses. Different tasks may have different physical addresses, thereby preventing the tasks from interfering with each other. * Cause a trap to the monitor by forcing a page fault on the memory-mapped page. Read-only pages trap writes. Not-present pages trap both reads and writes. Intervention for every I/O might be excessive for some kinds of I/O devices. A page fault can still be used in this case to cause intervention on the first I/O operation. The monitor can then at least make sure that the task has exclusive access to the device. Then the monitor can change the page status to present and read/write, allowing subsequent I/O to proceed at full speed.

15.5.3 Special I/O Buffers

15.5.3 Special I/O Buffers Buffers of intelligent controllers (for example, a bit-mapped graphics buffer) can also be virtualized via page mapping. The linear space for the buffer can be mapped to a different physical space for each virtual 8086 task. The V86 monitor can then assume responsibility for spooling the data or assigning the virtual buffer to the real buffer at appropriate times.

15.6 Differences from 8086

15.6 Differences From 8086 In general, V86 mode will correctly execute software designed for the 8086, 8088, 80186, and 80188. Following is a list of the minor differences between 8086 execution on the 80386 and on an 8086. 1. Instruction clock counts. The 80386 takes fewer clocks for most instructions than the 8086/8088. The areas most likely to be affected are: * Delays required by I/O devices between I/O operations. * Assumed delays with 8086/8088 operating in parallel with an 8087. 2. Divide exceptions point to the DIV instruction. Divide exceptions on the 80386 always leave the saved CS:IP value pointing to the instruction that failed. On the 8086/8088, the CS:IP value points to the next instruction. 3. Undefined 8086/8088 opcodes. Opcodes that were not defined for the 8086/8088 will cause exception 6 or will execute one of the new instructions defined for the 80386. 4. Value written by PUSH SP. The 80386 pushes a different value on the stack for PUSH SP than the 8086/8088. The 80386 pushes the value of SP before SP is incremented as part of the push operation; the 8086/8088 pushes the value of SP after it is incremented. If the value pushed is important, replace PUSH SP instructions with the following three instructions: PUSH BP MOV BP, SP XCHG BP, [BP] This code functions as the 8086/8088 PUSH SP instruction on the 80386. 5. Shift or rotate by more than 31 bits. The 80386 masks all shift and rotate counts to the low-order five bits. This MOD 32 operation limits the count to a maximum of 31 bits, thereby limiting the time that interrupt response is delayed while the instruction is executing. 6. Redundant prefixes. The 80386 sets a limit of 15 bytes on instruction length. The only way to violate this limit is by putting redundant prefixes before an instruction. Exception 13 occurs if the limit on instruction length is violated. The 8086/8088 has no instruction length limit. 7. Operand crossing offset 0 or 65,535. On the 8086, an attempt to access a memory operand that crosses offset 65,535 (e.g., MOV a word to offset 65,535) or offset 0 (e.g., PUSH a word when SP = 1) causes the offset to wrap around modulo 65,536. The 80386 raises an exception in these cases--exception 13 if the segment is a data segment (i.e., if CS, DS, ES, FS, or GS is being used to address the segment), exception 12 if the segment is a stack segment (i.e., if SS is being used). 8. Sequential execution across offset 65,535. On the 8086, if sequential execution of instructions proceeds past offset 65,535, the processor fetches the next instruction byte from offset 0 of the same segment. On the 80386, the processor raises exception 13 in such a case. 9. LOCK is restricted to certain instructions. The LOCK prefix and its corresponding output signal should only be used to prevent other bus masters from interrupting a data movement operation. The 80386 always asserts the LOCK signal during an XCHG instruction with memory (even if the LOCK prefix is not used). LOCK may only be used with the following 80386 instructions when they update memory: BTS, BTR, BTC, XCHG, ADD, ADC, SUB, SBB, INC, DEC, AND, OR, XOR, NOT, and NEG. An undefined-opcode exception (interrupt 6) results from using LOCK before any other instruction. 10. Single-stepping external interrupt handlers. The priority of the 80386 single-step exception is different from that of the 8086/8088. The change prevents an external interrupt handler from being single-stepped if the interrupt occurs while a program is being single-stepped. The 80386 single-step exception has higher priority that any external interrupt. The 80386 will still single-step through an interrupt handler invoked by the INT instructions or by an exception. 11. IDIV exceptions for quotients of 80H or 8000H. The 80386 can generate the largest negative number as a quotient for the IDIV instruction. The 8086/8088 causes exception zero instead. 12. Flags in stack. The setting of the flags stored by PUSHF, by interrupts, and by exceptions is different from that stored by the 8086 in bit positions 12 through 15. On the 8086 these bits are stored as ones, but in V86 mode bit 15 is always zero, and bits 14 through 12 reflect the last value loaded into them. 13. NMI interrupting NMI handlers. After an NMI is recognized on the 80386, the NMI interrupt is masked until an IRET instruction is executed. 14. Coprocessor errors vector to interrupt 16. Any 80386 system with a coprocessor must use interrupt vector 16 for the coprocessor error exception. If an 8086/8088 system uses another vector for the 8087 interrupt, both vectors should point to the coprocessor-error exception handler. 15. Numeric exception handlers should allow prefixes. On the 80386, the value of CS:IP saved for coprocessor exceptions points at any prefixes before an ESC instruction. On 8086/8088 systems, the saved CS:IP points to the ESC instruction itself. 16. Coprocessor does not use interrupt controller. The coprocessor error signal to the 80386 does not pass through an interrupt controller (an 8087 INT signal does). Some instructions in a coprocessor error handler may need to be deleted if they deal with the interrupt controller.

15.7 Differences from 80286 Real-Address Mode

15.7 Differences From 80286 Real-Address Mode The 80286 processor implements the bus lock function differently than the 80386. This fact may or may not be apparent to 8086 programs, depending on how the V86 monitor handles the LOCK prefix. LOCKed instructions are sensitive to IOPL; therefore, software designers can choose to emulate its function. If, however, 8086 programs are allowed to execute LOCK directly, programs that use forms of memory locking specific to the 8086 may not execute properly when transported to a specific application of the 80386. The LOCK prefix and its corresponding output signal should only be used to prevent other bus masters from interrupting a data movement operation. LOCK may only be used with the following 80386 instructions when they modify memory. An undefined-opcode exception results from using LOCK before any other instruction. * Bit test and change: BTS, BTR, BTC. * Exchange: XCHG. * One-operand arithmetic and logical: INC, DEC, NOT, and NEG. * Two-operand arithmetic and logical: ADD, ADC, SUB, SBB, AND, OR, XOR. A locked instruction is guaranteed to lock only the area of memory defined by the destination operand, but may lock a larger memory area. For example, typical 8086 and 80286 configurations lock the entire physical memory space. With the 80386, the defined area of memory is guaranteed to be locked against access by a processor executing a locked instruction on exactly the same memory area, i.e., an operand with identical starting address and identical length.