Hello everyone,

I just started recently with the Learn ASM in 28 Days Tutorial, and I've understood most of the basic stuff (binary, hex, base ten, basic commands, ROM calls, etc.) but I'm having a problem understanding what registers are and how they work.

Help? Please formulate your answer in the easiest possible way...I'm already having trouble with this

-lboe

Registers are your basic unit of data storage in asm. Basically, if you want to be doing something to data, you'll be using a register. If you're familiar with other programming languages, just think of them like a fixed set of variables. You use variables to hold data, and you use registers to hold data. Sometimes you need to hold more data, and then you use actual data sections, but you use the registers to point to the data (and hold bits of it as you go).

There are also some special registers, namely PC and SP. PC (program counter) holds the current location in the code where you're executing. So after each line of code it's incremented. Jumping modifies this value to point to a specific line. SP (stack pointer) is the current location in the stack, so when you push this is changed, and when you pop it's changed back to where it was before the push.

Thanks

So where would you store data ? Is it stored in specific memory addresses or just databytes (db.) as a lookup table ?

You can do either. The most common choice is to store data in the "safe" locations in RAM, eg. on the TI-83+, "appbackupscreen". A quick example of this:

Code:

This loads the register A with those values, and then saves them to the memory locations that are defined above. Noting that the register A is an 8-bit register, so to save it only requires 1 byte of RAM space.

The other .db method you mentioned is also an option, although this means permanently allocated memory, as the .db definitions take up space in your program (sometimes this has specific uses):

Code:


Hope that makes sense - I'm a terrible teacher!

Hardly a terrible teacher! Just to clarify something that people are often confused about: those #define lines aren't reserving RAM for you. Appbackupscreen is 768 bytes, and it remains 768 bytes no matter how much of it you use. All that the #define lines do is give locations within those 768 bytes other names, so you can refer to then directly. If you #define two different variables at the same address, they'd be the same variable (or always have the same value, if that's easier to think about).

Thats really easy to understand - thanks !