MS-DOS 1.51

According to MS-DOS 2.11 source code, MS-DOS 1.51 is an unreleased internal version of MS-DOS. At this stage of development, it is already clear that the next major release of MS-DOS is version MS-DOS 2.00 and it will differ significantly from the previous MS-DOS 1.25 release.

Change log
1.51 06/24/82 BREAKDOWN modified to do EXACT one sector read/write through system buffers