I got this far (not very)

MOV TH0,#HIGHVALUE0 ;load high value 
MOV TL0,#LOWVALUE0 ;load low value

And immediately said to myself "of what"

then

AGAIN: TIMER00 HIGH(DELAY_50ms),LOW(DELAY_50ms) ;call macro with high and low value of (-50000)

And said to myself "why delay 50ms"

Comments should reflect what is going on, why things are done, not what instructions are doing. If anyone looking at your code needed to "learn" what the instructions are doing (s)he should not look at the code.

Erik