Restore original message formatting
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Jeff Parsons
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1 changed files with 27 additions and 27 deletions
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@ -142,10 +142,10 @@ FOOTBALL Design Document
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---
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The following text is from an email titled "3xBox Design Document" sent to the
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*football* alias on Saturday, February 28, 1987, at 5:02pm.
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### Overview
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Microsoft *football* alias on Saturday, February 28, 1987, at 5:02pm.
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Overview
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--------
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The goal for this research project was to demonstrate the feasability of
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supporting multiple virtual DOS 3.x machines on a 286DOS-based kernel running
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on an 386 personal computer. Each "3xBox" would have its own virtual screen,
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@ -174,8 +174,8 @@ The following text is from an email titled "3xBox Design Document" sent to the
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Unless stated otherwise, most of the concepts extant in 286DOS apply to 386DOS.
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### V86 Mode and the 386
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V86 Mode and the 386
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----------------------
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The 386 CPU has three distinct execution modes: REAL, PROT, and V86. REAL
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and PROT modes are largely compatible with the corresponding modes of an 286.
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V86 modes is exactly the same as RING 3 PROT mode, with the following
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@ -203,8 +203,8 @@ The following text is from an email titled "3xBox Design Document" sent to the
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sensitive.
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This allows the OS to simulate the Interrupt Flag, if desired.
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### V86 IRETD Frame
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V86 IRETD Frame
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---------------
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When any interrupt, trap, exception, or fault occurs in V86 mode, the CPU
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switches to PROT mode and switches to the TSS Ring 0 Stack and builds the
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following stack frame:
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@ -219,8 +219,8 @@ The following text is from an email titled "3xBox Design Document" sent to the
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(0) (old CS)
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(old EIP) <- (SS:SP)
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### CPU Mode Determination
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CPU Mode Determination
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----------------------
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A new implementation of the WHATMODE macro was written in order to distinguish
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between the three CPU modes: REAL, PROT, and V86. REAL mode is indicated by
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a 0 PE bit in CR0 (a.k.a. MSW on a 286). If the PE bit is 1, then the mode
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@ -230,8 +230,8 @@ The following text is from an email titled "3xBox Design Document" sent to the
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be changed. So, we change IOPL and then check to see if it changed. If so,
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PROT mode, else V86 mode.
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### CPU Mode Switching
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CPU Mode Switching
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------------------
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The 286DOS kernel relies extensively on switching inbetween REAL and PROT.
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This functionality is provided by the RealMode and ProtMode routines.
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In 386DOS, RealMode is no longer needed. As soon as we switch to PROT mode
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@ -256,28 +256,28 @@ The following text is from an email titled "3xBox Design Document" sent to the
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the caller. NOTE: V86 Services is also used for completing the 386 LOADALL
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used by PhysToVirt to map "high" memory in "REAL" mode.
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### Stack Switching
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Stack Switching
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---------------
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In order to maintain the 286DOS mode switch and stack switch semantics
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when V86 mode is used, we have a new stack (the V86 Stack) in the 3xBox PTDA.
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### 286DOS Modes and Stacks
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286DOS Modes and Stacks
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-----------------------
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The RealMode and ProtMode procedures in 286DOS are the only ways to switch
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the CPU execution mode. These routines both maintain SS:SP, allowing
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RealMode and ProtMode to be reentrant. The TSS Ring 0 stack is always the
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current TCB stack in the current PTDA. The only other stacks in the system
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are the Interrupt Stack and user stack(s).
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### 386DOS Modes and Stacks
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386DOS Modes and Stacks
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-----------------------
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In 386DOS, any interrupt or exception while in V86 mode causes a switch to
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PROT mode and the TSS Ring 0 Stack. So we have a new way to mode switch with
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an incompatible stack semantic. We had to fix this mode switch to make it
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compatible with 286DOS.
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### Observation
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Observation
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-----------
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In V86 mode, the current stack must not be the TSS Ring 0 Stack. The CPU
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only leaves V86 mode via an interrupt/exception, which causes a stack switch
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to the TSS Ring 0 Stack. If the current stack was the same as the TSS Ring 0
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@ -285,8 +285,8 @@ The following text is from an email titled "3xBox Design Document" sent to the
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the PTDA. Since we run on this stack in V86 mode, we need a new Ring 0 stack
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when a 3xBox is running.
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### Approach
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Approach
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--------
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1) When a PMBox is running, the TSS Ring 0 Stack is a PTDA TCB stack.
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+ This is consistent with the 286DOS model.
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@ -298,8 +298,8 @@ The following text is from an email titled "3xBox Design Document" sent to the
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+ Otherwise, copy the V86 IRETD frame to the previous stack and
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switch back to the previous stack.
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### Details
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Details
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-------
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1) Leaving V86 mode
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a. V86ToProt (analog of ProtMode)
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+ Issue special V86ToProt software interrupt. If the interrupt
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@ -341,8 +341,8 @@ The following text is from an email titled "3xBox Design Document" sent to the
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+ Execute 386 LOADALL with VM bit set in EFLAGS image in loadall
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buffer.
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### Interrupt Management
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Interrupt Management
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--------------------
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All software interrupts, hardware interrupts, and CPU traps and exceptions
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are vectored through a common IDT, regardless of whether the CPU is in PROT
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or V86 mode.
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@ -352,8 +352,8 @@ The following text is from an email titled "3xBox Design Document" sent to the
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keyboard and mouse (since those are implicitly for the foreground box),
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can be given to background 3xBoxes.
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### Passing Hardware Interrupts to the Foreground 3xBox
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Passing Hardware Interrupts to the Foreground 3xBox
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---------------------------------------------------
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In the interrupt manager:
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IF a 3xBox is foreground -AND-
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