Unfortunately, the early DOS File Manager subverted this efficiency by copying bytes read from or written to a file one at a time between a disk buffer and main memory, requiring more time and resulting in DOS constantly blowing revs when reading or writing files.
12.
First, the DR DOS kernel and structures such as disk buffers can be located in the High Memory Area ( HMA ), the first 64 KB of extended memory which are accessible in real mode due to an incomplete compatibility of the 80286 with earlier processors.
13.
Later, programmers outside Apple rewrote the File Manager routines to avoid making the extra copy for most sectors of a file; RWTS was instructed to read or write sectors directly to or from main memory rather than from a disk buffer whenever a full sector was to be transferred.
14.
Starting with DOS 5, DOS could directly take advantage of the HMA by loading its kernel code and disk buffers there via the DOS = HIGH statement in CONFIG . SYS . DOS 5 + also allowed the use of available UMBs via the DOS = UMB statement in CONFIG . SYS.
