A86 (software)

A86 is computer software, a compact commercial assembler developed for the Intel x86 family of microprocessors by Eric Isaacson. It was first made available as shareware in the 1980s. The assembler is contained in one 32K executable and can directly produce an MS-DOS-compatible COM file or an object file for use with a standard linker. While still supporting expected x86 syntax, A86 allows the use of slightly simpler shorthand in some cases. It also does not require the use of directives, such as ASSUME and PROC, as do Microsoft Macro Assembler and other contemporaries. Isaacson claimed that A86 can assemble 100,000 lines of source per second on a Pentium II or better.[1] It comes with a debugger, D86.[2][3]

Developer(s)Eric Isaacson
Stable release
4.05 / 2006 (2006)
Operating systemMS-DOS

A86/D86 targets 16-bit x86 platforms. Isaacson added 32-bit support in the mid 1990s in the form of A386 and D386.[2][3] These were not distributed as shareware, but were provided to users who registered A86.

Isaacson never ported the A86 family of products to Windows, though it is possible to use the object files created by A386 with a Windows linker.[4] The assembler cannot be used to create 64-bit code.

A86-specific features

Any label that is one letter followed by one or more digits is local, and can be redefined as needed, with each definition replacing the prior. Isaacson recommends using L1 through L9 for normal code and M1 through M9 in macros. Branches to local labels are assumed to be backward, but a forward branch can be specified by prefixing the label reference with the greater than symbol (e.g.,>L2).

Numbers starting with a zero are hexadecimal, such as 00FF, instead of using a custom prefix.

Multiple registers can be pushed or popped on the same source line. push eax, edx, ebp generates one instruction for each of the three operands.

There's a shorthand for conditionally executing a single instruction, e.g., if z mov ax,bx. This generates two instructions: a conditional branch followed by the move.

If ret is specified as the label for a conditional branch, it automatically targets a return instruction within the 8-bit branch range (e.g., jz ret).

Code fingerprint

The assembler automatically embeds a "fingerprint" into the generated code through a particular choice of functionally equivalent instruction encodings. This makes it possible to tell if code was assembled with A86, and also to distinguish between registered and unregistered versions of the assembler, although access to the source code is required.[5]


  1. Isaacson, Eric (2006). "A86/A386 and D86/D386 features".
  2. Isaacson, Eric (2008). "A86/A386 assembler and D86/D386 debugger". Archived from the original on 2 July 2008. Retrieved 2008-07-02.
  3. Randall Hyde. "Which Assembler is the Best?". Archived from the original on 15 May 2008. Retrieved 2008-05-18.
  4. Isaacson, Eric (2006). "A386 and Win32 Programming".
  5. El-Khalil, Rakan; Keromyti, Angelos D. (2004). "Hydan: Hiding Information in Program Binaries" (PDF). In Lopez, Javier; Qing, Sihan; Okamoto, Eiji (eds.). Information and communications security: 6th International Conference, ICICS 2004, Malaga, Spain, October 27–29, 2004 : Proceedings. Lecture Notes in Computer Science. 3269. Springer. p. 190. ISBN 3-540-23563-9.CS1 maint: uses editors parameter (link)
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