What was the single byte change to port WordStar from CP/M to DOS? - history

Question

I was re-reading Joel's Strategy Letter II: Chicken and Egg problems and came across this fun quote:
In fact, WordStar was ported to DOS
by changing one single byte in the
code. (Real Programmers can tell you
what that byte was, I've long since
forgotten).
I couldn't find any other references to this with a quick Google search. Is this true or just a figure of speech? In the interest of my quest to become a "Real Programmer", what was the single byte change?

Answer 1

Sounds a bit exaggerated, found some WordStar history here
WordStar 3.0 for MS-DOS
Apr 1982
In one single all-night session Jim Fox patched the CP/M-86 version of WordStar to make it run under MS-DOS on the IBM PC so that it could be demonstrated to Rubenstein. The actual port was done by a group of Irish programmers using Intel development systems, which ran the ISIS II operating system. The software build was done on 8" floppies and the binary (executable) files were then transferred to the IBM PC by serial cable.
But...Joel maybe meant MS-DOS 1.0 / QDOS
MS-DOS 1.0 was actually a renamed version of QDOS (Quick and Dirty Operating System), which Microsoft bought from a Seattle company, appropriately named Seattle Computer Products, in July 1981. QDOS had been developed as a clone of the CP/M eight-bit operating system in order to provide compatibility with the popular business applications of the day such as WordStar and dBase. CP/M (Control Program for Microcomputers) was written by Gary Kildall of Digital Research several years earlier and had become the first operating system for microcomputers in general use.

Answer 2

You'd have to change more than one byte. CP/M-86 executables (.CMD) all have a 128-byte header, which isn't anything like the .EXE header.
If you restrict all your API calls to the common subset of CP/M and DOS, then you can use conditional assembly to build CP/M and DOS versions from the same source:
bdos:
if CPM86
int 0E0h
else
mov ah, cl
int 21h
endif

Answer 3

This Wikipedia entry claims that CP/M and MS-DOS share binary formats. It goes on to say:
Although the file format is the same
in MS-DOS and CP/M, this does not mean
that CP/M programs can be directly
executed under MS-DOS or vice versa;
MS-DOS COM files contain x86
instructions, while CP/M COM files
contain 8080, 8085 or Z80
instructions.
Under CP/M 3, if the first byte of a
COM file is 0xC9 then this indicates
the presence of a 256-byte header;
since 0xC9 corresponds to the 8080
instruction RET, this means that the
COM file will immediately terminate if
run on an earlier version of CP/M that
does not support this extension.
This implies that perhaps the fix/port was changing this first instruction into something else, that alowed the rest to execute. Not sure though, that seems to imply that the binary must have been "fat", which seems unreasonable for a legacy binary.

Answer 4

WordStar was written in 8080 assembler, and there were tools back then to convert 8080 to 8086 assembler (the 8086 instruction set was designed to allow this) if all the code could fit into a single segment, so this is quite possible.
I first used WordStar in 1979, on a Z80 CP/M box. People today might not realise how lucky they are - how many MS Word users would be prepared as the first task on installing their word processor to have to write a couple of small assembler routines (in hex!) to interface the word processor efficiently (you could use the CP/M routines but they were dog slow and didn't work properly) with the screen and keyboard? Happy days...

Answer 5

"In fact, WordStar was ported to DOS by changing one single byte in the code. (Real Programmers can tell you what that byte was, I've long since forgotten)."
Mon 6/08/2009 6:27 pm. My assumption was Spolsky was talking about 8080 CP/M to 8086 MSDOS, and that the story is probably bogus. 8086 CP/M was never a big item -- I mean, it was crushed utterly by MSDOS -- and someone may have converted WordStar from 8080-CPM to 8086-CPM -- by reassembling it, as others have noted, using a special 8080-to-8086 translator thingey -- and then perhaps only a single byte had to be changed.

Answer 6

I'm not sure whether Joel's statement is accurate or not. Perhaps he meant the demonstration version that Jim Fox made?
See http://www.wordstar.org/wordstar/history/history.htm
I'll quote the pertinent section:
WordStar 3.0 for MS-DOS
Apr 1982
In one single all-night session Jim
Fox patched the CP/M-86 version of
WordStar to make it run under MS-DOS
on the IBM PC so that it could be
demonstrated to Rubenstein. The actual
port was done by a group of Irish
programmers using Intel development
systems, which ran the ISIS II
operating system. The software build
was done on 8" floppies and the binary
(executable) files were then
transferred to the IBM PC by serial
cable.
(Edit: Oops, too late. Someone else already found the exact same thing :-/ Feel free to ignore me.)

Answer 7

An important thing to understand is that at the time 16-bit 8086 machines was just coming out to replace the current 8-bit machines, where the CP/M operating system was the Windows of the day. Everything with a disk drive intended for work ran CP/M. That version was later called CP/M-80 to differentiate it from CP/M-86 for the 8086 processor.
Unfortunately that took so long to get to market that QDOS was written to have SOMETHING to run programs on, and that was essentially a quick reimplementation of the CP/M functions (but with a different syntax). QDOS was later bought by Microsoft and made into MS-DOS. Hence MS-DOS actually has a CP/M core deep deep inside, and therefore the amount of work needed to get a CP/M-86 program to run under MS-DOS was limited (not to a single byte, but manageable).
I had the pleasure to work a few years with CCP/M-86 which allowed multitasking very similar to what Linux in text mode (with virtual consoles) allow today. Unfortunately it never caught on. Oh, well, we have Linux :)