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19 votes

How does the 8086 jump instruction work when the target address overflows?

Most of the time on x86, you have to consider segments and offsets separately. Opcode 0xE9 is a near jump, it only touches IP. The addition wraps around, so the target is the relative offset (as a ...
Stephen Kitt's user avatar
16 votes

What did it cost the 8086 to support unaligned access?

The Missing Angle It feels a bit like the question misses the most important point about the whole 8086 project over discussing implementation details: 8080/85 compatibility The 8086 was intended as a ...
Raffzahn's user avatar
  • 224k
10 votes

Undocumented ModR/M byte combinations

As part of his series about the 8086 micro code Ken Shirriff has just added a blog post where he analyses some of the mentioned cases: Undocumented 8086 instructions, explained by the microcode It ...
Raffzahn's user avatar
  • 224k
6 votes

Why are first four x86 General Purpose Registers named in such unintuitive order?

Agreeing with existing answers but being too long to shove into a comment, per a recent post on Ken Shirriff’s blog that traced the instruction set from the Datapoint 2200 through the 8008, 8080 and ...
Tommy's user avatar
  • 37.3k
4 votes

What is the difference between the different versions of the SID or SID86 debugger for the x86 PC that was shipped with the different DR-DOS versions?

Ok, I did a little in-depth research in the last couple of weeks. And I found this: SID 1.0 (Concurrent DOS 3.1, Release Date 1983) assembles and disassembles 8086 and 8087 opcodes. supports symbolic ...
Coder's user avatar
  • 1,068
3 votes

How does the 8086 jump instruction work when the target address overflows?

I am absolutely not an expert on x86 bare-metal stuff (I play in the 8-bit era) but I've read enough over the years for this to set my A20 spidey-sense tingling. Is this the wrap-around behavior of ...
Eight-Bit Guru's user avatar
2 votes

8251A UART outputs unexpected data

Sorry for wasting your time. As Justme said, it seems it was running in 64x mode, and I got some data without frame errors with 900 baud but it was still not the ASCII letters I was sending. So while ...
x95's user avatar
  • 151
2 votes

Most modern C compilers targeting DOS 8086, running on DOS 8086 (16-bit)

(As I'm not allowed to comment, I have to "answer".) Turbo C/C++ or Turbo Pascal for DOS can still be downloaded from Embarcadero. I have working links at
bttr's user avatar
  • 21

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