Searching for VME

Of all that is known about the secrets contained in the Supplement to the Pentium Processor User's Manual ("Appendix H"), nothing is guarded more closely than details of the Virtual Mode Extensions (VME) implemented in the Pentium processor and late-model Intel486's. Even when closely reading the Pentium manuals, it is possible that the reader doesn't notice that enhancements to virtual-8086 mode exist. Yet the Pentium Processor Family Developer's Manual, Volume 3 is not silent on the subject. There are at least 27 references to VME in the Pentium manual. In addition to these references, another good source of information is Intel's British VME Patent application, which is publicly available. With a good understanding of Virtual-8086 mode (v86 mode), one could infer most of the remaining details of VME solely from those 27 references. All that's needed to characterize the complete details is an understanding of v86 mode, a little ingenuity, experimentation, persistence, and no qualms about hitting the reset button to restart a frozen computer. (For those with $12,000 to spare, an In-Circuit-Emulator (ICE) would be helpful too.)

The need for VME.

When v86 mode was originally implemented, software writers found two main purposes for its use. 1) DOS memory managers, and 2) DOS sessions under a multitasking operating system (MTOS) (like a DOS box in Windows). Under a memory manager, DOS remains a sequential single-tasking operating system. Therefore, hardware resources don't need to be arbitrated;[1] IOPL-sensitive instructions don't need to restricted at all. Running a DOS session under Windows is quite different. Nearly all resources need to be restricted. The DOS session should not have direct access to the hardware with the ability to program any and every device. Nor should the DOS session have the ability to directly control the interrupt flag (IF) of the EFLAGS register. Virtual-8086 mode has support for restricting such access, through the use of IOPL to restrict IOPL-sensitive instructions which modify IF,[2] and the I/O permission bit map to restrict access to I/O ports. However, there are a few shortcomings with the standard v86 mode.

1. Setting IOPL to 3 provides the better performance than setting IOPL less than 3. This setting reduces the trapping overhead, but lets Virtual DOS Machines (VDMs) disable interrupts, a potential integrity problem for the whole system.[3]
2. IOPL must be set less than 3 when the OS needs to virtualize the interrupt flag. When a Virtual Device Driver (VDD) needs to simulate a hardware interrupt into a VDM, it must be able to detect when the VDM is interruptible. Therefore IOPL must be less than 3 so that the interrupt flag can be virtualized.[3] Since IOPL is less than 3, performance is significantly degraded by attempts to execute interrupt flag sensitive (IF-sensitive) instructions which always fault to the v86 monitor.
3. An operating system may allow a VDM to receive real (external) interrupts, or virtual interrupts. This is a policy decision made by the OS implementers. If a v86 task only receives virtual interrupts, then it can be demand-paged, whereby it is swapped out to disk when real memory is needed for other purposes. If the v86 task receives real (external) interrupts, then it cannot be demand-paged, since the interrupt handler may be paged out when the interrupt occurs. The OS may not have enough time to bring in the entire task before another interrupt occurs; likewise it would be too complicated and still too time consuming to bring in just the portion of the task which contains the interrupt service routine.[4]
4. All INT-n instructions cause a switch out of v86 mode. When IOPL<3, an INT-n faults to the v86 monitor. When IOPL=3, the INT-n attempts to invoke the protected mode interrupt handler associated with that particular interrupt (success depends on the DPL of the gate being used). The monitor or interrupt handler must either emulate the interrupt's functionality, or restart the v86 task in such that it executes the interrupt itself (this is known as reflecting the interrupt). DOS kernel routines are accessed through software interrupts. Therefore, thousands of interrupt calls generate thousands of transitions in and out of v86 mode. This gives the v86 task a substantial performance disadvantage to the same program running under DOS.

VME fixes v86 problems

Enhanced v86 mode was designed to eliminate many of these problems, and significantly enhance the performance of v86 tasks running at all IOPL levels. When running in Enhanced virtual-8086 mode (Ev86) at IOPL=3, CLI and STI still modify IF. This behavior hasn't changed. Running at IOPL<3 has changed. CLI, STI, and all other IF-sensitive instructions no longer unconditionally fault to the Ev86 monitor. Instead, IF-sensitive instructions clear and set a virtual version of the interrupt flag in the EFLAGS register called VIF.[5] Clearing VIF does not block external interrupts, as clearing IF does. Instead, IF-sensitive instructions clear and set a virtual version of the interrupt flag called VIF. VIF does not control external interrupts as IF does. Rather, VIF is an indicator of the interruptibility state of the EV86 task. Thus, the operating system is invulnerable to a bug in a DOS program which inadvertently attempts to disable interrupts and spin inside of a loop, as VIF will be cleared instead, while IF will remain set. This new behavior has some substantial benefits: performance is increased, as CLI and STI don't cause time-consuming faults. Secondly, the complexity of the monitor program is reduced, as it doesn't have to maintain its own virtual interrupt flag. When the old-style v86 monitor virtualized IF, it needed to emulate all changes to IF caused by IF-sensitive instructions (CLI, STI, PUSHF, POPF, INT, and IRET). Using Ev86 mode eliminates this complexity because the CPU automatically virtualizes IF; performance increases because IF-sensitive instructions don't fault to the Ev86 monitor.

When external interrupts are generated, such as timer ticks and keyboard strokes, the host operating system running at CPL-0 always intercepts these interrupts. When some interrupts occur, the current task may not be the v86 task, it may be swapped out to disk, or it may be in an uninterruptible state. When this occurs, the host OS must delay sending the interrupt to the v86 task until it is running, and ready to accept interrupts. Other interrupts may be intended for a specific VDM, but not all VDMs (like keystrokes). In this case, the v86 monitor needs to send a specific interrupt to a specific VDM -- ignoring all other VDMs. Delaying and filtering interrupts in this manner is known as interrupt virtualization. Once the VDM with a virtual interrupt pending becomes interruptible, the OS reflects the interrupt to the VDM as if a real interrupt had occurred.

Prior to Ev86 mode, the v86 monitor needed to maintain a virtual interrupt flag in software. The v86 monitor was forced to handle many exceptions which were unnecessary. For example, when a virtual interrupt was pending, further IOPL-sensitive instructions which attempted to clear IF caused undesired faults, which then caused the monitor to redundantly clear the virtual interrupt flag. This problem doesn't exist in Ev86 mode. These instructions which redundantly attempt to clear IF don't fault to the monitor. Therefore the source code which exists to clear the software virtual interrupt flag can be removed. In fact, while using Ev86 mode, all of the code needed to maintain the software virtual interrupt flag can be removed -- as the virtual interrupt flag is maintained by the CPU itself.

Prior to Ev86 mode, software interrupts (INT-n instructions) always caused a switch out of v86 mode. If IOPL=3, the transition occurs through a gate associated with the interrupt;[6] when IOPL<3, the transition occurs as the result of a general protection fault to the monitor. When IOPL=3, the monitor needs to determine whether or not the cause of the interrupt is a software interrupt, external interrupt or CPU-generated exception. When IOPL<3, software interrupts don't transition through their associated gates in the IDT (they transition through the #GP gate). In the case of software interrupts, the monitor must interpret the opcode to determine which interrupt number needs servicing. The monitor must then emulate the interrupt, or reflect the interrupt back to the v86 task. External interrupts and CPU-generated exceptions still transition through their associated gate in the IDT. For these cases, the monitor still needs to determine the source of the interrupt (external or CPU-exception), and take the appropriate action. Using Ev86 mode can simplify this process, and enhance the performance of handling software interrupts.

Software interrupt execution is controlled by a new structure in the TSS called the interrupt redirection bit map (IR bit map). Each bit in this new structure controls whether or not a specific software interrupt will be invoked in a manner compatible with the Intel386, or be invoked purely within the Ev86 task. In Ev86 mode, these interrupts may be invoked and executed without ever leaving the Ev86 task. Using this new technique would reduce complexity in the monitor. Interrupts which would normally fault to the monitor, no longer would. Interrupts which would transition through the IDT no longer would.

Overview of VME Components

VME support is enabled and disabled by setting and clearing the VME bit in CR4 (bit 0). When enabled and running at IOPL=3, all INT-n instructions are controlled by the interrupt redirection bit map in the TSS.[7] When running at IOPL<3, in addition to the INT-n behavior, IF-sensitive instruction are allowed to execute without faulting to the Ev86 monitor.

The TSS has been extended to include a 32-byte interrupt redirection bit map. 32-bytes is exactly 256 bits, one bit for each software interrupt which can be invoked via the INT-n instruction. This bit map resides immediately below the I/O permission bit map (see Figure 1). The definition of the I/O Base field in the TSS is therefore extended and dual purpose. Not only does the I/O Base field point to the base of the I/O permission bit map, but also to the end (tail) of the interrupt redirection bit map. This structure behaves exactly like the I/O permission bit map, except that it controls software interrupts. When its corresponding bit is set, an interrupt will fault to the Ev86 monitor. When its bit is clear, the Ev86 task will service the interrupt without ever leaving Ev86 mode.

Figure 1 -- Interrupt redirection bit map in the TSS

VIF and VIP EFLAGS Bits

Two new flags were added to the EFLAGS register. These flags are intended for use when the IOPL of the Ev86 task is less than 3 (see sidebar Caveats Of VME (When CR4.VME=1)). They can only be purposely modified by the CPL-0 Ev86 monitor or an interrupt service routine.

VIF is a virtualized version of the standard interrupt flag (IF). While the Ev86 task is running, any CLI and STI instruction will not modify the actual IF, instead these instructions modify VIF.[5] This fact is completely hidden from the Ev86 task, as PUSHF, POPF, INT-n, and IRET have also been modified to help hide this behavior.

The VIP flag is a Virtual Interrupt Pending flag. VIP can assist the multitasking operating system in sending a virtual interrupt to the Ev86 task. The easiest way to understand VIP is to explain its use in the context of a program running on an 8086. When the 8086 is in an uninterruptible state, external interrupts remain pending but don't get serviced. After IF is set (because of STI, POPF, or IRET), the pending interrupt is serviced by the CPU. VIF and VIP are intended to serve this same purpose to the MTOS running an Ev86 task. Let's assume your Ev86 task was at the same uninterruptible point as the previous 8086 example. A timer-tick interrupt occurs, and the MTOS services the interrupt. During the interrupt service routine, the MTOS decided that the Ev86 task needs to service this timer tick, and sets VIP. After returning, the Ev86 task is still in an uninterruptible state (VIF=0). At some later time, the Ev86 task attempts to set IF (STI, POPF, or IRET). When this happens, the Ev86 task becomes interruptible, and a general protection fault to the monitor immediately occurs (#GP(0)).[8]

IF-sensitive instructions

To support the new VIF and VIP flags, changes were needed to the instructions which read and write the interrupt flag of the EFLAGS register. CLI, STI, PUSHF, POPF, INT-n, and IRET all had to be changed to support Ev86 mode.

When an Ev86 task is running at IOPL<3, CLI, and STI clear and set the VIF flag, instead of faulting to the Ev86 monitor, or affecting the IF flag.[5]

PUSHF copies the contents of the VIF flag to the IF position as it pushes the FLAGS image onto the stack. This gives the appearance to the Ev86 task that STI and CLI are really setting and clearing IF. This appearance is necessary in case the software attempts to check for this condition. Such a code sequence which tests for the interrupt flag, is seen in Listing 1. In addition to moving the VIF to the IF on the stack image, PUSHF always pushes an IOPL image of 3 onto the stack. It is important to remember that the Pentium's IF-sensitive instructions behave identically to the Intel486 when IOPL=3, even when CR4.VME=1. Therefore, PUSHF simulates an IOPL=3 to any software wishing to read the stack image to determine its IOPL. The actual IOPL of the Ev86 task never changes during this process.

Listing 1 -- Code demonstrating how software tests for the IF flag

STI                             ; Enable interrupts
PUSHF                           ; Store FLAGS on stack
POP     AX                      ; Restore flags into register
TEST    AX,200h                 ; Interrupt flag set?
Jcc     label                   ; Jump on condition

POPF works similar to PUSHF by copying the bit in the IF position to VIF flag as it pops the FLAGS image from the stack. The Pentium is careful to make sure that the faked IF and IOPL aren't accidentally copied into the real IOPL during the POPF operation. Before the FLAGS image is merged into the EFLAGS register, the IF image is copied to the VIF slot, the IF and IOPL images are cleared. All of the actual FLAGS register bits are cleared, except the actual IF and IOPL. Finally, the filtered FLAGS image is merged with the actual EFLAGS register. A side-effect of POPF is its handling of the TF in the stack image. If the TF on the stack image is set, then POPF causes a general protection fault before any FLAGS values are modified (#GP(0)).

The IRET instruction behaves exactly as the POPF instruction does with respect to IF, VIF and IOPL. IRET and POPF differ in how they handle the trap flag from the stack image. If TF is set in the FLAGS stack image during POPF, a #GP(0) occurs, yet for IRET the #GP does not occur.

The INT-n instruction is the most complicated of the IOPL-sensitive instructions. INT-n behaves exactly like PUSHF in how it handles IF, VIF, and IOPL.[9] However, one of the enhancements to Ev86 mode is the ability of the Ev86 task to execute software interrupts without leaving Ev86 mode. This enhancement has been accomplished with the aid of the interrupt redirection bit map in the TSS. When the corresponding IR bit is set, the interrupt will be invoked in exactly the same manner as a normal v86 task. When the corresponding bit is clear, the interrupt is invoked as if it were executing on an 8086 processor. In other words, a fault to the monitor is never generated, nor a transition to the protected mode interrupt handler. The interrupt transition and return are done entirely within the Ev86 task. The influence of the IR bit map is best described by the pseudo-code in Listing 2.

Listing 2 - Interrupt handling description in Ev86 mode

N = INTERRUPT_NUMBER;
INTERRUPT_BIT_MAP_PTR = TSS_BASE->IO_PERMISSION_BASE - 32;
IF INTERRUPT_BIT_MAP_PTR->BIT_NUMBER[N]
    IF (IOPL<3)
        #GP(0);
    ELSE
        GOTO INT-FROM-V86-MODE;
ELSE
    INVOKE_REAL_MODE_STYLE_INTERRUPT_FROM_Ev86_TASK(N);

Conclusions

The virtual mode extensions are very useful to memory managers and multitasking operating systems. Memory managers can primarily benefit by the use of the interrupt redirection bit map to reduce the number of switches to and from protected mode. This has the added benefit of reducing the complexity of the interrupt service routines, as they no longer have to reflect software interrupts back to the v86 task.

Multitasking operating systems can benefit in many ways. The MTOS benefits from interrupt redirection, and from the virtual interrupt support. The MTOS would run with virtual mode extensions enabled, and the Ev86 tasks running at IOPL<3. This gives the MTOS full benefit of the virtualization of interrupts. When the MTOS wishes to send a virtual interrupt (like a virtual timer-tick) to an uninterruptible Ev86 task, it will do so by setting VIP=1. When the task becomes interruptible, a general protection fault occurs, and the MTOS will send the virtual interrupt to the Ev86 task. This would give programs which are timer-dependent (such as games) a significant performance advantage. As an added benefit of using the virtualization features of the CPU, even more complexity of the Ev86 monitor can be removed. The result of using these new features, is an Ev86 monitor that is simpler to implement and maintain than its non-Ev86 counterpart, and software which runs faster.

Endnotes

1. Except I/O ports subject to I/O protection from the I/O permission bit map (like DMA ports).
2. The I/O port instructions which are IOPL-sensitive in protected mode are not IOPL-sensitive in v86 mode.
3. The Design of OS/2 by H.M. Deitel & M.S. Kogan; Chapter 10 "Compatibility - 80386 DOS Compatibility" Section 10.4.1.
4. The Design of OS/2 by H.M. Deitel & M.S. Kogan; Chapter 10 "Compatibility - 80386 DOS Compatibility" Section 10.4.
5. STI will fault when EFLAGS.VIP=1.
6. Assuming that all other protection attributes will permit the transition to occur.
7. Interrupt redirection is not conditional upon the IOPL setting.
8. See sidebar Caveats Of VME (When CR4.VME=1).
9. Provided this interrupt is redirected from the Ev86 monitor to the Ev86 task (subject to the Interrupt Redirection bit map).
   

Source code examples 1:

The following examples are available for viewing and download.

The first eight examples demonstrate how to reflect an interrupt back to the (E)v86 task in various processor environments.

                         DPL    INTR
             VME  IOPL  IGATE  BITMAP
VME_1.ASM     0    3      0       X
VME_2.ASM     0    2      X       X
VME_3.ASM     0    3      3       X
VME_4.ASM     1    3      0       1
VME_5.ASM     1    2      X       1
VME_6.ASM     1    3      3       1
VME_7.ASM     1    3      X       0
VME_8.ASM     1    2      X       0

View source code for VME_1.ASM:
ftp://ftp.x86.org/source/vme1/vme_1.asm
ftp://ftp.x86.org/source/vme1/struct.inc
ftp://ftp.x86.org/source/vme1/macros.inc
ftp://ftp.x86.org/source/vme1/sysseg.inc
ftp://ftp.x86.org/source/vme1/dataseg.inc

View source code for VME_2.ASM:
ftp://ftp.x86.org/source/vme1/vme_2.asm

View source code for VME_3.ASM:
ftp://ftp.x86.org/source/vme1/vme_3.asm

View source code for VME_4.ASM:
ftp://ftp.x86.org/source/vme1/vme_4.asm

View source code for VME_5.ASM:
ftp://ftp.x86.org/source/vme1/vme_5.asm

View source code for VME_6.ASM:
ftp://ftp.x86.org/source/vme1/vme_6.asm

View source code for VME_7.ASM:
ftp://ftp.x86.org/source/vme1/vme_7.asm

View source code for VME_8.ASM:
ftp://ftp.x86.org/source/vme1/vme_8.asm

Download all eight examples and executables:
ftp://ftp.x86.org/dloads/VME1.ZIP

Source code examples 2:

The second source code example demonstrates the handling of VIF in Ev86 mode. This was written by Jim Brooks and serves as an excellent programming example. There are too many files included to provide links to each and every one of them, so click here to download the entire source code archive, or you may view individual files at:

ftp://ftp.x86.org/source/v86mon1

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