Code_ The Hidden Language of Computer Hardware and Software - Charles Petzold [138]
CP/M was once a very popular operating system for the 8080 and remains historically important. CP/M was the major influence behind a 16-bit operating system named QDOS (Quick and Dirty Operating System) written by Tim Paterson of Seattle Computer Products for Intel's 16-bit 8086 and 8088 chips. QDOS was eventually renamed 86-DOS and licensed by Microsoft Corporation. Under the name MS-DOS (Microsoft Disk Operating System, pronounced em ess dahs, like the German article das), the operating system was licensed to IBM for the first IBM Personal Computer, introduced in 1981. Although a 16-bit version of CP/M (called CP/M-86) was also available for the IBM PC, MS-DOS quickly became the standard. MS-DOS (called PC-DOS on IBM's computers) was also licensed to other manufacturers who created computers compatible with the IBM PC.
MS-DOS didn't retain CP/M's file system. The file system in MS-DOS instead used a scheme called the File Allocation Table, or FAT, which had been originally invented at Microsoft in 1977. The disk space is divided into clusters, which—depending on the size of the disk—can range in size from 512 bytes to 16,384 bytes. Each file is a collection of clusters. The directory entry for a file indicates only that file's starting cluster. The FAT itself indicates for each cluster on the disk what the next cluster is.
The directory entries on an MS-DOS disk are 32 bytes long and use the same 8.3 filenaming convention as CP/M. The terminology is a little different, however: The last three letters are called the filename extension rather than the file type. The MS-DOS directory entry need not contain a list of allocation blocks. Instead, the directory includes such useful information as the date and time the file was last modified, and the size of the file.
The early versions of MS-DOS were structured much like CP/M. But the BIOS wasn't required in MS-DOS because the IBM PC itself included a complete BIOS in ROM. The command processor in MS-DOS is a file named COMMAND.COM. MS-DOS programs come in two flavors. Programs with the filename extension COM are limited to 64 KB in size. Larger programs have the filename extension EXE (pronounced eks-ee, for executable).
Although MS-DOS initially supported the CALL 5 interface for API functions, a newer interface was recommended for new programs. The newer interface used a feature of the 8086 called the software interrupt, which is similar to a subroutine call except that the program doesn't need to know the actual address that it's calling. A program calls an MS-DOS API function by executing the instruction INT 21h (pronounced int twenty—one, even though it's hexadecimal).
In theory, application programs are supposed to access the hardware of the computer only through the interfaces provided by the operating system. But many application programmers who dealt with small computer operating systems of the 1970s and early 1980s often bypassed the operating system, particularly in dealing with the video display. Programs that directly wrote bytes into video display memory ran faster than programs that didn't. Indeed, for some applications—such as those that needed to display graphics on the video display—the operating system was totally inadequate. What many programmers liked most about MS-DOS was that it "stayed out of the way" and let programmers write programs as fast as the hardware allowed.
For this reason, popular software that ran on the IBM PC often relied upon idiosyncrasies of the IBM PC hardware. Manufacturers of machines intended to be competitive with the IBM PC were often forced to duplicate these idiosyncrasies;