59
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 ...
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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 ...
- 4,796
40
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 ...
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37
votes
Accepted
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 ...
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35
votes
Accepted
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 ...
- 118k
35
votes
Accepted
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 ...
- 118k
34
votes
Accepted
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,...
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34
votes
Accepted
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 ...
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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 ...
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27
votes
Accepted
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 ...
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26
votes
Accepted
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 ...
- 6,119
20
votes
Accepted
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 ...
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19
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 ...
- 213k
15
votes
Accepted
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.
...
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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'...
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14
votes
Accepted
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 ...
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14
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 ...
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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 ...
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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 ...
- 1,274
12
votes
Accepted
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 ...
- 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 ...
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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 ...
- 118k
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 ...
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12
votes
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 ...
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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 ...
- 118k
11
votes
Accepted
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 ...
- 213k
11
votes
Accepted
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 ...
- 118k
11
votes
Accepted
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 ...
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11
votes
Were Windows 3.x applications dependent upon 80286 instructions?
While Win 3.x operating system stuck to the 80286 platform,
Not really as Windows was since 2.1 available in two versions:
Windows/286 and
Windows/386
It got only unified by name with Windows 3.0. ...
- 213k
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