ORG 0000H
MOV R0,#00H
MOV R1,#50H
MOV R2,#00H
MOV R3,#60H
MOV R4,#05H
MOV DPL,R0 LOOP:
MOV DPH,R1
MOVX A,@DPTR
CPL A
MOV DPL,R2
MOV DPH,R3
MOVX @DPTR,A
INC R0
INC R2
DJNZ R4,LOOP
END

Can anybody help me in understanding this assembly code of 8051?

This is my first rant here, so I hope everyone has a good time reading it.

So, the company I am working for got me going on the task to do a rewrite of a firmware that was extended for about 20 years now. Which is fine, since all new machines will be on a new platform anyways. (The old firmware was written for an 8051 initially. That thing has 256 byte of ram. Just imagine the usage of unions and bitfields...)

So, me and a few colleagues go ahead and start from scratch.
In the meantime however, the client has hired one single lonely developer. Keep in mind that nobody there understands code!
And oh boy did he go nuts on the old code, only for having it used on the very last machine of the old platform, ever! Everything after that one will have our firmware!

There are other machines in that series, using the original extended firmware. Nothing is compatible, bootloaders do not match, memory layouts do not match, code is a horrible mess now, the client is writing the specification RIGHT NOW (mind, the machine is already sold to customers), there are no tests, and for the grand finale, the guy canceled his job and went to a different company. Did I mention the bugs it has and the features it lacks?

Guess who's got to maintain that single abomination of a firmware now?

Get replaced by an AI^WDeep ML device. That's coded for a 8051 and running on an emulator written in ActionScript, being executed on a container so trendy its hype hasn't started yet, on top of some forgotten cloud.
Then get called in to debug my replacement.