reece@enuxha.eas.asu.edu (Glen A. Reece) (06/03/90)
Greetings again from AZ:
I was having a discussion the other day with a friend who asked about
writing applications for the Mac. I told him that the limitation to
application size was not memory but the 32K static data barrier that exists
on the Mac (64K on the PC I think). Anyway, can someone please give a
good explanation of the reason for the 32K limit and reason as to why Apple
has not eliminated this constraint.
Please respond to me directly, and if others are interested then I'll
post the responses back to comp.sys.mac
Thankx in advance,
- Glen A. Reece
Industrial Fellow
Artificial Intelligence Lab.
Arizona State Universitydoner@henri.ucsb.edu (John Doner) (06/04/90)
In article <894@enuxha.eas.asu.edu> reece@enuxha.eas.asu.edu (Glen A. Reece) writes: >Greetings again from AZ: > > I was having a discussion the other day with a friend who asked about >writing applications for the Mac. I told him that the limitation to >application size was not memory but the 32K static data barrier that exists >on the Mac (64K on the PC I think). Anyway, can someone please give a >good explanation of the reason for the 32K limit and reason as to why Apple >has not eliminated this constraint. > There is no 32K barrier; i.e., there is nothing intrinsic to the Macintosh or its operating system which leads to such a limitation. Nevertheless, one does encounter it, so your puzzlement is justified. The Motorola 68000 series chips have several addressing options for their instructions, for example the branching instructions have forms with 8 or 16 bit displacements from the current value of the program counter, or a full 32 bit address. Data references allow for similar variations. The shorter forms reduce code size, and were especially important at a time when memory was more expensive than it is today. As a result, many compilers gave 16-bit integers a special role, and used them for array subscripts. This leads to a 32K limitation on the array size. Apple designed its jump tables with 16 bit displacements from the beginnings of code segments, so externally referenced routines must appear in the first 32K bytes of a code segment. Within the segments, the 16-bit offset branching instructions are used, thus limiting segment size to a practical maximum of 32K. For the most part, these limitations are compiler-dependent, and disappear when the compiler designers work to eliminate them. (The jump table limitation isn't just a matter of generating different instructions, but it too can be overcome.) Language Systems Fortran and Absoft Fortran are two examples of compilers without 32K limitations. Since Apple doesn't control the compiler writers, and there is a need to maintain compatability with earlier products, they are limited in what they can do to get away from 32K limitations. They are clearly headed in that direction. ----------- John E. Doner | "The beginner...should not be discouraged if...he Mathematics, UCSB | finds that he does not have the prerequisites for Santa Barbara, CA 93106| reading the prerequisites." doner@henri.ucsb.edu | --Paul Halmos, Measure Theory