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60 votes

How does the LOADALL instruction on the 80286 work?

LOADALL is an undocumented instruction available on the 80286, 80386 and some 486 clones. It provides a means to load all the CPU's registers in one operation, including normally-inaccessible ...
Stephen Kitt's user avatar
44 votes
Accepted

What conventions and language extensions did people use to program the 8086 and 80286?

What it boils down to is that, unlike in a flat addressing model, there are multiple types of pointers: near - 16-bit (16-bit offset only.) far - 32-bit (16-bit offset + 16-bit segment) huge - The ...
mschaef's user avatar
  • 4,836
41 votes

How did the Apple II forward binary instructions to the Z80 software card with CPM?

They both shared the same memory so it didn't really forward instructions. The Z80 card stopped the 6502 running using the DMA signals and the system swapped between the two by writing to $CN00 where ...
PeterI's user avatar
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37 votes
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How to switch an 80286 from protected to real mode?

I’m aware of three main techniques used to exit protected mode on 286s; they all involve resetting the CPU, they vary on how that is done: using the keyboard controller (which IBM endowed with the ...
Stephen Kitt's user avatar
36 votes
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Replacing 80286 with 68000

No, there is no simple one-to-one mapping for the pins. (Bolded signal names will be active-low.) For example, while the 286 has two physical pins for interrupts (INTR and NMI), 68000 has three (IPL0,...
telcoM's user avatar
  • 726
35 votes
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What was this OS / Application on a 80286?

Since the computer was a gift from your aunt, working for IBM, the screen split in four reminds me immediately of the IBM PS/1’s “4-quadrant” interface: (The screenshot above is from IBMulator, an ...
Stephen Kitt's user avatar
35 votes
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How much slower was the 286 in protected mode?

Basically, anything that involves changing segments is slower, sometimes significantly so; this is unsurprising since descriptors have to be checked, privilege levels potentially changed etc. Other ...
Stephen Kitt's user avatar
34 votes
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Were Windows 3.x applications dependent upon 80286 instructions?

Windows 3.x wasn’t “stuck to the 80286 platform”: Windows 3.0 still supported real mode on 8086 processors. The requirement for a 286 at the platform level only came with Windows 3.1. As far as ...
Stephen Kitt's user avatar
29 votes

80286 can switch from real mode to protected mode - but why not back?

My guess is that it was merely a design decision based upon the assumption that once a protected mode OS is started, there is no need to go back. Most microprocessors at that time already booted in ...
mcleod_ideafix's user avatar
28 votes
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80286 can switch from real mode to protected mode - but why not back?

This was intentional so that the CPU would support secure operating systems. In a secure operating system with rigorous memory access protections you could not allow any software - user or kernel ...
davidbak's user avatar
  • 6,289
28 votes
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Did any software attempt to perform 8086 emulation on the 80286 using LOADALL, in the vein of the later virtual 8086 mode of the 80386?

I believe what you are describing was in fact done. Concurrent DOS for the 286 could multitask DOS programs in protected mode. See the Wikipedia article on Multiuser DOS as well as DOS VMs. The ...
RETRAC's user avatar
  • 13.7k
20 votes

Replacing 80286 with 68000

Most obvious question first: why not puting itn on a ISA Card and take over the bus instead? Given, there would be still some work to be done after asking for DMA and pulling /MASTER, but way less ...
Raffzahn's user avatar
  • 224k
19 votes
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Which operating systems for 80286 computers allowed a process to use more than 128k data?

OS/2 supported “huge memory” on 286s. The New Executable format used for 16-bit OS/2 executables (and 16-bit Windows executables) supports multiple segments. At runtime, using the DosAllocHuge ...
Stephen Kitt's user avatar
15 votes
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How did the Apple II forward binary instructions to the Z80 software card with CPM?

TL;DR: this longish answer address the "mystic" property of the question; i.e., the sense of wonder about how this could be possible; not the actual workings of the specific components. The gory ...
AnoE's user avatar
  • 1,579
15 votes
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Is a Heatsink Needed for an Intel 80286?

The 80286 has a maximum power dissipation of 3.3W and an operating range of up to 70°C. Unless you want to operate near that point, no heat sink is needed. The ICs surface is alreaady way too large. ...
Raffzahn's user avatar
  • 224k
15 votes

80286 real mode emulator for 8086

There are 386 real-mode emulators for 286s, such as Eko Priono’s EMU386; but they relied on one important feature which 8086s don’t have, the invalid instruction exception. Whenever a 286 attempts to ...
Stephen Kitt's user avatar
14 votes

How can I access an old pre-IDE disk?

That Conner CP-343 drive appears to be a special snowflake, but it's not. It's actually a standard IDE drive, with one exception. That 20-pin interface you're looking at isn't the IDE connector; it'...
mnem's user avatar
  • 4,545
12 votes

What conventions and language extensions did people use to program the 8086 and 80286?

That's a lot of parts for a single question. •what conventions did programmers employ to deal with segmentation? •what extensions exist for common programming languages to accommodate the segmented ...
peter ferrie's user avatar
  • 1,314
12 votes

80286 pricing and availability

PC Magazine 11/1984 was first reporting about the PC/AT 5170. They quote a price tag of 3800 USD for the low end configuration with 256 KiB and 5800 USD for the high end configuration with 512 KiB RAM ...
Arne's user avatar
  • 1,274
12 votes
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How do you put a 286 in Protected Mode?

Actually this is a lot easier than I thought, after trying to link to another MSW note, I found it in the Intel Instruction Set: Machine Status Word (286+ only). The machine status word seems to be a ...
Evan Carroll's user avatar
  • 3,388
12 votes

How to switch an 80286 from protected to real mode?

The trick is that resetting the CPU leaves the RAM intact (unless the system designers have done something that will clear some or all RAM on a reset). So if you leave appropriate information in the ...
cjs's user avatar
  • 26.6k
12 votes

0F1h opcode-prefix on i80286

According to an Intel document describing opcodes which don’t result in exception 6, The 0F1H opcode is a prefix which performs no function. It counts like any other prefix towards the maximum ...
Stephen Kitt's user avatar
12 votes

Did any software attempt to perform 8086 emulation on the 80286 using LOADALL, in the vein of the later virtual 8086 mode of the 80386?

As RETRAC writes, this was indeed done. Companies other than Digital Research also considered similar approaches, but ultimately discarded them. One documented case is IBM with OS/2’s DOS support ...
Stephen Kitt's user avatar
12 votes
Accepted

How is FSTSW AX implemented on the 80286/80287?

Since this is about low level operation, let's start with the fact that the CPU/FPU does not provide an FSTSW AX instruction, only an FNSTSW AX. When encountering FSTSW AX, the assembler issues two ...
Raffzahn's user avatar
  • 224k
11 votes
Accepted

What was the fastest third party FPU?

I don’t recall SPARC systems having separate sockets for discrete FPUs; in particular, the Weitek SPARC POWER µP was a replacement SPARC CPU which derived much of its speed benefit from doubling its ...
Stephen Kitt's user avatar
11 votes
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First 80286 Based Computer

Well, one of the first will be for sure Intels System 310/286. According to the CHM this would be 1981, but I think it was rather 1982/83 - that would also fit the original flyer. It's a Multibus ...
Raffzahn's user avatar
  • 224k
11 votes
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Did the PC/AT-bus use its expanded address space?

Yes, there were 16-bit ISA cards which used the extra address lines; for example IBM’s various memory expansion options, most of which could be configured to provide extended memory beyond 1MiB. You ...
Stephen Kitt's user avatar
10 votes
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0F1h opcode-prefix on i80286

While it is officially documented as "a prefix which performs no function", a thread on the vcfed.org forums has discovered it is an ICE-mode prefix that forces the instruction's memory access to be ...
Philip's user avatar
  • 351
10 votes

80286 real mode emulator for 8086

Have you ever thought of changing the 8088 in your PC to a NEC V20 (uPD70108)? The V20 is basically a 80186/286 EU with an 8088 BU. This offers all the 'new' real mode instructions (*1) you need, ...
Raffzahn's user avatar
  • 224k

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