MS-DOS 1.25 is one of the earliest versions of the MS-DOS operating system. It was released in August 1982.
The source code for this version was released on March 25th, 2014 by Microsoft and the Computer History Museum, along with a note from Tim Paterson from December 2013:
From: Tim Paterson
To: Len Shustek
Date: Mon, 16 Dec 2013 10:34:17 -0800
Subject: RE: Source code to MS-DOS 1.0
I have found and attached the source code for MS-DOS 1.25 as shipped by Seattle Computer Products. Version 1.25 was the first general release to OEM customers other than IBM so was used by all the first clone manufacturers.
IBM's DOS 1.1 corresponds to MS-DOS 1.24. There is one minor difference between 1.24 and 1.25, as noted in the revision history at the top of MSDOS.ASM.
Of the file attached, only STDDOS. ASM/MSDOS.ASM (DOS main code) and COMMAND.ASM (command processor) would have been used by an OEM other than Seattle Computer. The other files:
IO.ASM - I/O system unique to SCP (equivalent to ibmbio.sys). ASM.ASM & HEX2BIN.ASM - Old 8086 assembler developed by SCP (used to assemble older version of DOS). TRANS.ASM - Z80 to 8086 assembly source code translator developed by SCP.
I also have a 6” stack of printouts of assembly listings for some of these and probably other related programs.
Zenith OEM (1.20)