{"id":69,"date":"2022-12-28T11:56:00","date_gmt":"2022-12-28T11:56:00","guid":{"rendered":"https:\/\/thebrokenpipe.com\/blog\/?p=69"},"modified":"2026-04-26T09:54:16","modified_gmt":"2026-04-26T09:54:16","slug":"porting-86-dos-1-14-to-the-ibm-pc","status":"publish","type":"post","link":"https:\/\/thebrokenpipe.com\/blog\/porting-86-dos-1-14-to-the-ibm-pc\/","title":{"rendered":"Porting 86-DOS 1.14 to the IBM PC"},"content":{"rendered":"\n
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<\/path><\/svg>2024 Edit<\/p>\n

Following the discovery of other versions of 86-DOS and newer efforts, this port is now outdated<\/u>. The latest ports of 86-DOS to the IBM PC are on GitHub<\/a> and binaries may be downloaded from there too.

Nonetheless, this post is still worth reading because it forms the basis of my newer ports and provides context for some of my later posts.<\/p>\n<\/div>\n\n\n\n

A friend challenged me to compile and boot MS-DOS 1.25 on a modern computer. But, I decided to do something slightly more challenging. Instead of MS-DOS, I decided to go with the old and proprietary 86-DOS, which never supported the IBM PC line of computers and does not have source code available. There are only two versions of 86-DOS out in the wild – 1.00 and 1.14, and I rolled with 1.14 for the task.<\/p>\n\n\n

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86-DOS 1.14 booting on a modern laptop.<\/figcaption><\/figure>\n<\/div>\n\n\n\n\n\n\n

Why not 86-DOS 1.00? Well, it has already been done by somebody else (I’ll talk about it later).<\/p>\n\n\n\n

PC-DOS 1.00 BIOS & Boot Sector Disassembly<\/h2>\n\n\n\n

Before I began porting 86-DOS, I created binary-exact disassemblies<\/a> of PC-DOS 1.00’s IBMBIO.COM<\/a> and boot sector<\/a> to learn about the real-life uses of the PC BIOS interrupts. Instead of rewriting the aforementioned components from scratch, I ended up recycling the disassemblies for this port.<\/p>\n\n\n\n

Getting the Files<\/h2>\n\n\n\n

The copy of 86-DOS 1.14<\/a> on Bitsavers<\/a> is in the IBM 8-inch single-sided single-density 250.25 KiB format. Nothing can read FAT partitions with 128-byte sectors and the CP\/M-style reserved system area, so I extracted the files manually. Luckily, the FAT implementation is the same as the MS-DOS\/PC-DOS one and directory entries are 32 bytes in length, so it didn’t take me long to extract all 21 files.<\/p>\n\n\n\n

The Files & Their Fate<\/h2>\n\n\n\n

Some files were modified\/patched to work on the PC, some remained unchanged while others were simply removed\/replaced.<\/p>\n\n\n\n

File<\/th>Description<\/th>Fate<\/th><\/tr><\/thead>
86DOS.SYS<\/code><\/td>86-DOS Kernel<\/td>Patched to work on the PC.<\/td><\/tr>
ASM.COM<\/code><\/td>8086 Assembler<\/td>Standard DOS program, unchanged.<\/td><\/tr>
BOOT.ASM<\/code><\/td>Boot Sector Source Code<\/td>Replace with (a modified copy of) PC-DOS 1.00’s boot sector source code.<\/td><\/tr>
CHKDSK.COM<\/code><\/td>Disk Check Utility<\/td>Standard DOS program, unchanged.<\/td><\/tr>
COMMAND.COM<\/code><\/td>Command Interpreter<\/td>Standard DOS program, unchanged.<\/td><\/tr>
CPMTAB.ASM<\/code><\/td>RDCPM<\/code> Config File<\/td>Modified to suit the reading of 5.25″ CP\/M-86 disks.<\/td><\/tr>
DATE.COM<\/code><\/td>DATE Utility<\/td>Standard DOS program, unchanged.<\/td><\/tr>
DEBUG.COM<\/code><\/td>Debugger<\/td>Standard DOS program, unchanged.<\/td><\/tr>
DOSIO.ASM<\/code><\/td>BIOS Source Code<\/td>Replaced with (a modified copy of) PC-DOS 1.00’s BIOS source code.<\/td><\/tr>
EDLIN.COM<\/code><\/td>Line Editor<\/td>Standard DOS program, unchanged.<\/td><\/tr>
HEX2BIN.COM<\/code><\/td>Intel Hex to Binary Converter<\/td>Standard DOS program, unchanged.<\/td><\/tr>
INIT.ASM<\/code><\/td>Disk Initialiser Source Code<\/td>Extensively modified (basically rewritten from scratch) to format 5.25″ single-sided double-density disks on the PC.<\/td><\/tr>
INIT.COM<\/code><\/td>Disk Initialiser<\/td>Recompiled from the modified source code.<\/td><\/tr>
MAKRDCPM.COM<\/code><\/td>RDCPM Configuration Utility<\/td>Standard DOS program, unchanged.<\/td><\/tr>
MON.ASM<\/code><\/td>SCP 8086 Monitor ROM Source Code<\/td>Removed, that ROM chip does not exist on the IBM PC motherboard.<\/td><\/tr>
NEWS.DOC<\/code><\/td>RDCPM<\/code> and INIT<\/code> Notes<\/td>Updated to reflect the changes I made.<\/td><\/tr>
RDCPM.COM<\/code><\/td>CP\/M Disk Reader<\/td>Patched (recompiled) to read 5.25″ CP\/M-86 disks instead of 8″ CP\/M-80 disks.<\/td><\/tr>
READTHIS.DOC<\/code><\/td>Addendum for 86-DOS 1.1<\/td>Updated to reflect the changes I made to the operating system.<\/td><\/tr>
SYS.COM<\/code><\/td>System Transfer Utility<\/td>Extensively modified (basically rewritten) to be compatible with the ported OS.<\/td><\/tr>
TIME.COM<\/code><\/td>TIME Utility<\/td>Standard DOS program, unchanged.<\/td><\/tr>
TRANS.COM<\/code><\/td>Z80 to 8086 Translator<\/td>Standard DOS program, unchanged.<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

Of these 21 files:<\/p>\n\n\n\n