DOS ain't dead

Question about some x86 opcodes (Developers)

posted by mht , Wroclaw, Poland, 05.11.2008, 19:54

Things are not that clear... Microsoft people say:

http://support.microsoft.com/kb/69987
For optimization reasons, MASM may generate the opcode 83 for logical AND, OR, and XOR instructions in some cases, rather than opcode 81. Unfortunately, opcode 83 was not documented by Intel for 80x86/8088 processors prior to the 80386. Therefore, some processors (such as the NEC V25 and V35 controllers) and some in-circuit emulators for the 80x86 family do not support this opcode.

and NEC Electronics people say (in V25 and V35 documentation):

http://www.necel.com/cgi-bin/nesdis/o006_e.cgi?article=UPD70330
Even in this case, instructions are executed normally. Take precautions, however, since some emulators do not support the disassembly function or line assembly function for this instruction.

(many thanks to Lucho for both links).

Then I looked into PC-DOS 5.00, MS-DOS 6.22, PC-DOS 7.10 and DR-DOS 7.03 kernel binaries. Microsoft's and IBM's kernels contain mainly the "short" forms (a few "long" forms may be hypothetically explained by the assembler not knowing the value in the first pass, particularly that the non-problematic instructions also happen to be "longer than needed"). DR-DOS kernel contains both forms, probably because of two different assemblers used for different source files (MASM and Digital Reasearch's RASM86) -- at least both are needed to build OpenDOS 7.01.

NEC V20 chips (V25 is a microcontroller version of it) were often used in "Turbo-XT" machines as a faster replacement of Intel 8088. Did anyone ever complained about DOS not running correctly on those? I doubt. Also much of other software would experience problems there. So I think that NOSIGNEXTEND is not necessary on PCs, unless some emulators or debuggers are considered.

Complete thread:

• Question about some x86 opcodes - mht, 02.11.2008, 14:02 (Developers)
  • Question about some x86 opcodes - Japheth, 02.11.2008, 17:51
  • Question about some x86 opcodes - DOS386, 03.11.2008, 10:04
    • Question about some x86 opcodes - mht, 05.11.2008, 19:54
      • Question about some x86 opcodes - Rugxulo, 06.11.2008, 21:06
        • Question about some x86 opcodes - mht, 08.11.2008, 08:46
          • Question about some x86 opcodes - Rugxulo, 12.11.2008, 00:43
            • Question about some x86 opcodes - mht, 12.11.2008, 16:54
              • Question about some x86 opcodes - mht, 22.11.2008, 13:59