Nowadays it is quite easy to find any kind of information on the internet.

But when I was a kid it was a mission to get valid information.

You could go to the library – at first – but as this was a valid choice for mathematical & etc stuff, things were not that easy for very technical stuff or computer-related topics.

You can also find some good books that are not available at the library because too specific (but you need to find a library where you can order such books, and you mainly need the money to do so). Or you can wait to see a good article in some magazines (there were so much good computer magazines at this time!).

You can also experiment things by yourself and try to find answers, but it takes a lot of time and you end up running out of evidence …

And another one I was doing a lot is to exchange information with others, note things, keep papers organized or store them on disks. People you meet at coding parties, people that share the same love and are eager to share and get some emulation out of it!

 

But in the end, this can result in false information…

And one belief I had is concerning MOVEQ.

 

The ‘quick’ version of MOVE, ADD,  SUB have the data information directly encoded in the opcode of the instruction. How neat! Allowing it to be fast decoded and executed.

I always thought this could be done for values from 0 to 7 (8 values in total), such as:

ADDQ.W #3, D1
SUBQ.B #1, D3
MOVEQ.L #0, D0

But once you get the real information about the 68000 opcodes. You see that this is true for ADDQ, SUBQ where the data is encoded in the opcode with 3 bits (hence the 8 possible values) but not for the MOVEQ where the data is coded on 8 bits!

Thus something like:

MOVEQ.L #$FF, D0

is possible!!

If only I knew this before!! I could have made some more optimization here and there.

That’s why this is so priceless to me the fact you can find a lot of information for free and I sometimes wonder how we managed to deliver things in the past…
Just traveling back in time and provide me with an opcode grid for the 68000 could have saved me lots of hours and nights!!