M/CP v2.0

It's back! Version 2 of the Minimal Control Program is here! This time with all new updated features! Welcome to the world of
unfriendly computing with M/CP, the smallest operating system you didn't know you needed!
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Whats new?

The M/CP project has now developed into a truly small control program that fits onto the bootsector of any diskette, 
with over 40 bytes to spare! Even better, with 5 built-in programs, this bootsector control program can display and edit whats 
in memory, read and write from any BIOS visible drives to and from memory, and finally transfer control to any location in 
memory! And once you're done, should you wish to return to the Minimal Control Program, all you need to do is call the one API 
function, Int 20h to transfer back to the control program command line! Of course with the standard caveats of it being a real 
mode operating system (i.e. please dont overwrite the area between 0000:7C00h and 0000:7E00h or hook Int 20h badly).
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System requirements

Running M/CP wont be too much of a problem for most users since 1983. 
The minimum requirements are: 
- An IBM PC or with PC with a compatible BIOS
- A minimum of 32Kb of RAM (due to the loading of the boot sector at 07C00h)
- An MDA/CGA/EGA/VGA or compatible card
- A diskette drive (by default, the provided .BIN file will work on 5.25" 360Kb Floppy drives, you will need to reassemble for 
  non 5.25" Floppy drives. Details below).
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Feature guide

Five programs are included with M/CP:
-(d)ump

-(e)dit

-(l)oad

-(w)rite

-(j)ump

To use these programs, you must type the program name (indicated by the lowercase letter in the brackets) followed by a space 
(mandatory, you will not be allowed to proceed until the space key is pressed) and then proceed to type the command line 
arguments for each function as described below. Please note, if you make a mistake you are free to use the backspace key to 
delete entered values, though this might not necessarily be reflected in the command line. This will be rectified.
If you type a key that is not a control key (Enter, space, backspace) or a lowercase hexadecimal digit, the computer will beep 
and will reset. This can be used also to reset the machine in the event of a mistake in typing in the command line.
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Guide to internal programs:

Below you will see a reference guide on how to use the control programs' internal commands. Each argument is separated by a 
space and is indicated as follows: [Usage,x] with Usage indicating what the meaning of the value that should go there is, and x 
indicating up to how many digits this value can be. Remember all values are given in hexadecimal with alphabetical digits
given in lowercase!!

Please note that for the load and write programs, I use an LBA addressing scheme that addresses CHS=(0,0,0) as LBA=0.

The DUMP program.
On the command line type the following:
d [Segment, 4] [Offset, 4]

The EDIT program.
On the command line type the following:
e [Segment, 4] [Offset, 4] [Repleacement byte, 2]

The LOAD program.
On the command line type the following:
l [Segment, 4] [Offset, 4] [BIOS Drive byte code, 2] [LBA number, 4] [Number of sectors to read, 2]

The WRITE program.
On the command line type the following:
w [Segment, 4] [Offset, 4] [BIOS Drive byte code, 2] [LBA number, 4] [Number of sectors to write, 2]

The JUMP program.
On the command line type the following:
j [Segment, 4] [Offset, 4]

If you wish to enter the entry 0000 for any of the entries, you MAY just press the space key. Also, you MUST type in ALL the
digits as indicated, otherwise numbers will be shifted. I leave it to you, the user, to figure out how they are shifted! 

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Guide to API:

M/CP has a simple API. There is one function.

Int 20h - Return back to the operating system. There is no exit code passed by this function but on the plus side, you dont 
need to be in the same segments Int 20h, as the function handles the setting or resetting of the segment registers itself.
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Warning to users of non 5.25" floppy drives:

The program contains a BPB (and an extended BPB which may be removed in future versions) to maintain a FAT readable diskette 
or drive even whilst having this program in the bootsector. Hence, because this was written for and tested on a 5.25" drive,
the BPB values are designed for it. 

If you wish to use it on a non 5.25" drive or drive image, YOU MUST CHANGE THE BPB variable sectrc (sectors/track) to the 
appropriate number for your media. For 1.44Mb 3.5" diskettes, for example, sectrc becomes 0012h. This can be done by hex-editing 
the binary file or reassembling from the the source. 
If you choose to reassemble, please change the variable sectrc from the word 0009h to 0012h. If you hex edit simply change the 
word at sector offset 18h (BPB offset 0Dh) from 0009h to 0012h.

Please then reassemble the .asm file as needed. I may provide a 3.5" version in the future.
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Final notes:

The source code is heavily commented, so it should be fairly easy to understand how it works.
I'm more than happy to answer any questions about the code and my other projects. You can contact me via my reddit account, u/ylli122 
or via email (although, try not to spam me please), at [email protected]
Many thanks to everyone that's supported me so far!

PS. There is a lot of commented and manually encoded commands in the source code. This was done a bit out of desperation and a bit just because it is a quick fix since MASM 2.0 wasn't assembling the instructions correctly. I'll look into correcting this 
later as well as a few nicites. I am also considering adding a FAT driver but we shall see...

This was written on an IBM PC XT with 256Kb of RAM, assembled and linked with MASM and LINK 2.0.