4 ms·

I think the 286 was designed way before any of that was a consideration. It was first released in 1982, just a couple of months after the IBM PC.

pjmlp 3y ago
From its Wikipedia article, "The problems led to Bill Gates famously referring to the 80286 as a "brain-dead chip",[25][when?] since it was clear that the new Microsoft Windows environment would not be able to run multiple MS-DOS applications with the 286. It was arguably responsible for the split between Microsoft and IBM, since IBM insisted that OS/2, originally a joint venture between IBM and Microsoft, would run on a 286 (and in text mode). Other operating systems that used the protected mode of the 286 were Microsoft Xenix (around 1984),[26] Coherent,[27] and Minix.[28] These were less hindered by the limitations of the 80286 protected mode because they did not aim to run MS-DOS applications or other real-mode programs. In its successor 80386 chip, Intel enhanced the protected mode to address more memory and also added the separate virtual 8086 mode, a mode within protected mode with much better MS-DOS compatibility, in order to satisfy the diverging needs of the market."
rasz 3y ago
386 got trapping privileged accesses wrong, not fixed in 486. Problem is Protected Mode programs (CPL 3) executing CLI and expecting POPF to restore interrupt flag - its ignored AND doesnt trap. Afaik no x86 CPU ever fixed it completely. Microsoft had some hacks trying to detect and patch known common problem patterns: Getting MS-DOS games to run on Windows 95: The interrupt flag https://devblogs.microsoft.com/oldnewthing/20160411-00/?p=93281 https://devblogs.microsoft.com/oldnewthing/20160411-00/?p=93... Virtualbox switches to software tricks: https://docs.oracle.com/en/virtualization/virtualbox/6.0/admin/swvirt-details.html https://docs.oracle.com/en/virtualization/virtualbox/6.0/adm... "In particular, the recompiler is used when guest code disables interrupts and Oracle VM VirtualBox cannot figure out when they will be switched back on. In these situations, Oracle VM VirtualBox actually analyzes the guest code using its own disassembler." ReactOS didnt even try. "ReactOS' NTVDM on the other hand, is a software emulator just like DOSBox": https://web.archive.org/web/20170723164052/http://community.reactos.org/index.php/news/years-progress-ntvdm-part-1 https://web.archive.org/web/20170723164052/http://community.... https://web.archive.org/web/20151216085857/http://community.reactos.org/index.php/news/years-progress-ntvdm-part-2 https://web.archive.org/web/20151216085857/http://community.... https://xem.github.io/minix86/manual/intel-x86-and-64-manual-vol3/o_fe12b1e2a880e0ce-996.html https://xem.github.io/minix86/manual/intel-x86-and-64-manual... "When the PVI flag is set to 1, the CPL is 3, and the IOPL is less than 3, the STI and CLI instructions set and clear the VIF flag in the EFLAGS register, leaving IF unaffected. In this mode of operation, an application running in protected mode and at a CPL of 3 can inhibit interrupts in the same manner as is described in Section 20.3.2, “Class 2—Maskable Hardware Interrupt Handling in Virtual-8086 Mode Using the Virtual Interrupt Mechanism”, for a virtual-8086 mode task. When the application executes the CLI instruction, the processor clears the VIF flag. If the processor receives a maskable hardware interrupt, the processor invokes the protected-mode interrupt handler. This handler checks the state of the VIF flag in the EFLAGS register. If the VIF flag is clear (indicating that the active task does not want to have interrupts handled now), the handler sets the VIP flag in the EFLAGS image on the stack and returns to the privilege-level 3 application, which continues program execution. When the application executes a STI instruction to set the VIF flag, the processor automatically invokes the general-protection exception handler, which can then handle the pending interrupt. After handing the pending interrupt, the handler typically sets the VIF flag and clears the VIP flag in the EFLAGS image on the stack and executes a return to the application program. The next time the processor receives a maskable hardware interrupt, the processor will handle it in the normal manner for interrupts received while the processor is operating at a CPL of 3." Pentium got Protected Mode Virtual Interrupts to half ass mitigate some of the symptoms, but doesnt fix the problem. https://www.felixcloutier.com/x86/popf:popfd:popfq https://www.felixcloutier.com/x86/popf:popfd:popfq "The protected-mode virtual-interrupt feature — enabled by setting CR4.PVI — affects the CLI and STI instructions in the same manner as the virtual-8086 mode extensions. POPF, however, is not affected by CR4.PVI"

pjmlp 3y ago

From its Wikipedia article, "The problems led to Bill Gates famously referring to the 80286 as a "brain-dead chip",[25][when?] since it was clear that the new Microsoft Windows environment would not be able to run multiple MS-DOS applications with the 286. It was arguably responsible for the split between Microsoft and IBM, since IBM insisted that OS/2, originally a joint venture between IBM and Microsoft, would run on a 286 (and in text mode). Other operating systems that used the protected mode of the 286 were Microsoft Xenix (around 1984),[26] Coherent,[27] and Minix.[28] These were less hindered by the limitations of the 80286 protected mode because they did not aim to run MS-DOS applications or other real-mode programs. In its successor 80386 chip, Intel enhanced the protected mode to address more memory and also added the separate virtual 8086 mode, a mode within protected mode with much better MS-DOS compatibility, in order to satisfy the diverging needs of the market."

rasz 3y ago

386 got trapping privileged accesses wrong, not fixed in 486. Problem is Protected Mode programs (CPL 3) executing CLI and expecting POPF to restore interrupt flag - its ignored AND doesnt trap. Afaik no x86 CPU ever fixed it completely. Microsoft had some hacks trying to detect and patch known common problem patterns: Getting MS-DOS games to run on Windows 95: The interrupt flag https://devblogs.microsoft.com/oldnewthing/20160411-00/?p=93281 https://devblogs.microsoft.com/oldnewthing/20160411-00/?p=93... Virtualbox switches to software tricks: https://docs.oracle.com/en/virtualization/virtualbox/6.0/admin/swvirt-details.html https://docs.oracle.com/en/virtualization/virtualbox/6.0/adm... "In particular, the recompiler is used when guest code disables interrupts and Oracle VM VirtualBox cannot figure out when they will be switched back on. In these situations, Oracle VM VirtualBox actually analyzes the guest code using its own disassembler." ReactOS didnt even try. "ReactOS' NTVDM on the other hand, is a software emulator just like DOSBox": https://web.archive.org/web/20170723164052/http://community.reactos.org/index.php/news/years-progress-ntvdm-part-1 https://web.archive.org/web/20170723164052/http://community.... https://web.archive.org/web/20151216085857/http://community.reactos.org/index.php/news/years-progress-ntvdm-part-2 https://web.archive.org/web/20151216085857/http://community.... https://xem.github.io/minix86/manual/intel-x86-and-64-manual-vol3/o_fe12b1e2a880e0ce-996.html https://xem.github.io/minix86/manual/intel-x86-and-64-manual... "When the PVI flag is set to 1, the CPL is 3, and the IOPL is less than 3, the STI and CLI instructions set and clear the VIF flag in the EFLAGS register, leaving IF unaffected. In this mode of operation, an application running in protected mode and at a CPL of 3 can inhibit interrupts in the same manner as is described in Section 20.3.2, “Class 2—Maskable Hardware Interrupt Handling in Virtual-8086 Mode Using the Virtual Interrupt Mechanism”, for a virtual-8086 mode task. When the application executes the CLI instruction, the processor clears the VIF flag. If the processor receives a maskable hardware interrupt, the processor invokes the protected-mode interrupt handler. This handler checks the state of the VIF flag in the EFLAGS register. If the VIF flag is clear (indicating that the active task does not want to have interrupts handled now), the handler sets the VIP flag in the EFLAGS image on the stack and returns to the privilege-level 3 application, which continues program execution. When the application executes a STI instruction to set the VIF flag, the processor automatically invokes the general-protection exception handler, which can then handle the pending interrupt. After handing the pending interrupt, the handler typically sets the VIF flag and clears the VIP flag in the EFLAGS image on the stack and executes a return to the application program. The next time the processor receives a maskable hardware interrupt, the processor will handle it in the normal manner for interrupts received while the processor is operating at a CPL of 3." Pentium got Protected Mode Virtual Interrupts to half ass mitigate some of the symptoms, but doesnt fix the problem. https://www.felixcloutier.com/x86/popf:popfd:popfq https://www.felixcloutier.com/x86/popf:popfd:popfq "The protected-mode virtual-interrupt feature — enabled by setting CR4.PVI — affects the CLI and STI instructions in the same manner as the virtual-8086 mode extensions. POPF, however, is not affected by CR4.PVI"