Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!att!chinet!edlee From: edlee@chinet.chi.il.us (Edward Lee) Newsgroups: comp.sys.ti Subject: Addressable memory Message-ID: <8958@chinet.chi.il.us> Date: 15 Jul 89 20:03:39 GMT References: <4752@psuvax1.cs.psu.edu> <1643@csm9a.UUCP> <176@zip.eecs.umich.edu> <5507@pt.cs.cmu.edu> <1657@csm9a.UUCP> Reply-To: edlee@chinet.chi.il.us (Edward Lee) Organization: Chinet - Public Access Unix Lines: 53 In article <1657@csm9a.UUCP> kamann@csm9a.UUCP (kamann) writes: >Is the Atari REALLY accessing X megabytes? As I recall most architectures >nowadays are built around a page-swapping facility, even IBM. Perhaps this is true. Perhaps the number of machines that use page- swapping outnumber the number of machines that do not. But for the past two years, I have regularly read flames every few months from other programmers complaining about Intel's segmented architectures in the 8088, 8086 chips. The 68000-based Atari computers can definitely address more than 8 megabytes of memory without page-swapping due to the fact that the 68000 CPUs have 32-bit registers. >Now, I know that IBM is designed around the concept of a 64k paging >area (or the XTs were anyway). So I think it would be safe to say >that an IBM only has 64k addressable also, or is it?!? Intermediate conclusions based on incomplete evidence tend to be transitory. It is safe to say that IBM PC-compatibles using the 8088 or 8086-style of memory handling have a maximum LINEAR addressable memory of 64KB. However, strictly speaking, IBM PC-compatibles generally have more than 64KB of addressable memory (note the absence of LINEAR). >Now, is the Amiga/Atari designed around the same concept? This question is ambiguous, with the ambiguity centering around the word SAME. >I wasn't aware that they had 20 bit buses, but there is still a limitation >to the amount of memory you can actually address, even BIG machines use >page swapping mechanisms and virtual memory. The width of a data bus does not by itself limit the amount of memory a CPU can address. You could have a computer with a 1 bit bus and a 16 bit CPU and still have the computer linearly address 64KB of memory, though the bus would be slow (serial). Unix and Vax-like computers use page swapping because they have memory protection, i.e. some amount of protection from memory over-runs by different processes. They may also be physically limited by the hardware architecture. Generally, virtual memory is any memory that is both based on non-semiconductor media (e.g. magnetic disks, optical disks, holographic plates) and is addressed as if it were CPU memory. Virtual memory is generally cheaper and larger -- but slower -- than comparable physical memory, so virtual memory is used for largely economical reasons. The counterpart to virtual memory is virtual disk, which is CPU memory used as if it were magnetic disk memory (i.e. RAMdisk). -Ed L (A student, looking for a summer job.)