Jim Seymour: Utilities: System Setup – The Inside Story; Setup and Configuration: Defining the Terms. In: PC Magazine. Band10, Nr.6. Ziff Davis, 26. März 1991, S.311, Defining Terms; continued (englisch, eingeschränkte Vorschau in der Google-Buchsuche): Himem area High memory area or HMA.
Barry Simon: Lab Notes – How to Get the Most From your System’s High DOS Memory. In: PC Magazine. Band9, Nr.10. Ziff Davis, 29. Mai 1990, S.347ff., HIMEM (englisch, eingeschränkte Vorschau in der Google-Buchsuche): “Microsoft did not invent HIMEM, as is usually supposed. It was used earlier by Quarterdeck’s DESQview, though it became widely available only with the release of Microsoft Windows 2.10. HIMEM uses a mere 64K of available extended memory—or, to be more exact, it uses the first 64K (minus 16 bytes) of extended memory, the memory directly above 1MB. The very important thing about HIMEM is that this 64K is accessible in real mode and can be used without a DOS extender. One possible use for HIMEM is to allow DOS to locate some of itself there, thus freeing up additional memory below 640K for user programs. HIMEM actually depends on a bug in the same way the 286 emulates the 8086! In addition to breaking up memory into pages—16 nonoverlapping 64K chunks—the CPU thinks of memory as overlapping 64K chunks, called segments, which start every 16 bytes. Suppose the CPU tries to access one of those segments that starts very close to the top of the 1MB boundary. An 8086 will wrap around to very low memory, as shown in Figure 2. On an 80286, however, the address actually accessed is just above the 1MB boundary. This provides real-mode addressability to the first 64K (minus 16 bytes) of extended memory. It’s 16 bytes less than 64K because the last segment begins 16 bytes shy of the 1MB boundary. To overcome the emulation bug, IBM built special capabilities into the bus that force memory to wrap around, like an 8086/88, in real mode. What the HIMEM driver does is to undo this safeguard selectively, so that a program specially written to use that area can put code there. That is what Microsoft Windows, DESQview, Ventura Publisher and Carousel do.”
