The code that lived in my head rent free for 30 years

Meanwhile:

    var
      regs: Registers;
    begin
      with regs do

Overall, I'd say the author fell in between the computer generations. After things started to look high-level, but where a lot of the interesting stuff was still very low-level.

One of the first things I learned to do in Commodore-64 basic was poke53280,0 to change screen background color. It was low level for everything, and there was no expectation of the address or the value "making sense" as such; it was just hardware control.

Were the files so big that it would have used too many floppies? Did the two computers use different sizes of floppies? Could he not afford blank floppies (but he could afford damaging his hard disk)? Did he not have the right software to split a file into pieces to fit on multiple floppies?

AH is the high byte of AX a 16 bit register in the 8086. AL is of course the low-byte of AX.

$13 is the hex number of the interrupt used in MSDOS.

What is AH or the Intr() call?

AH is as above. Intr() call is a Pascal call (was it Turbo Pascal, I wonder) to the 8086 interrupt code in a PC.

Yes this knowledge is arcane, and Yes, you 'just have to know it'

That's because this was the only way (Such a kluge!) you could interface the hardware to a "high level language" like Pascal that didn't have that capability built in.

It would probably have been better to use 8086 assembly with MSDOS system calls than Pascal as it was intrinsically able to interface with the hardware where Pascal was not.

But when you're a learner, you do what you can. Even if it turns out to be 'the hard way'.

We all have stuff that lives rent-free in our heads. In my case I can look at a lot of 8-bit hex numbers and know what machine instructions they represent in 8080/Z80 code. Enough of them to look at a hex dump of 8080 machine-code and know roughly what is supposed to happen.