I am storing my Scores like this:

Code:

Let's say this is my code before those 5 lines:

Code:


My understanding of the code:

Code:


I'm confused.

Code:
This is where you're wrong.

Consider what happens here:

• Load an address into HL. Let's say it's 0x8000.
  
• Get the byte at (HL) and store it into E. (0x8000) -> E.
  
• Get the byte at (HL) and store it into D. (0x8000) -> D.

See the problem yet?

Will work correctly:
Code:


This is incidentally something the index registers are handy for (though cases where it's actually more efficient to do so are rare).
Code:

(A bit off-topic, but i thought i'd share in anycase)

In this instance it's not very efficient to use the index registers, but if you've got a lot of data per pointer, for example a list of enemies with an x and y location, x and y velocity, id, animation frame, action id, etc. putting the address into ix can be really handy, particularly since you rarely touch them so you don't generally have to worry about it getting changed. I often use ix for handling bullets and enemies. Here's something from one of my games:

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If i'm messing with enemies, i do something like ld ix,enemy_array. If i want to read the x or y values, i can just do ld a,(ix+EX) or ld a,(ix+EY). If hit the enemy, i can do dec (ix+EHP). If the enemy is dead, i can ld (ix+EID),$FF. The best part is that i don't have to keep track how far i am in hl and when i've reached the end of the necessary stuff for that enemy i can just add ix,ENEMY_SIZE,

Back on topic, in some assembly languages (like ARM) you can have the register automatically increase when you read from it. The z80 doesn't, so if you want to read the next byte, you have to increase every time.

If you wanted to display the list of scores, for example, you would have something like this:

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...though you'd probably need to put hl and bc on the stack since disp_score will most likely change their values.

@chickendude
I tried using your code and modified the code to load Scores into ix instead and load the data from the ix address to h and l instead. It keeps loading the first value each time. I don't understand what is going on here.

Code:

I fixed the problem. I'm getting an Overflow error though when trying to display a number that is $00,$00 aka 0.

I'm using:

Code:

Fixed it. I'm an idiot and didn't load the # of digits to display into A before BCALL(_DispOP1A).

One thing you should keep in mind is that the z80 is little-endian, this means that the right part of the number gets stored first. Let's look at your scores:

Code:
The first score is of course zero, the second score is actually 256. This is because $00 is actually the LSB (least significant byte), $01 is the MSB (most significant byte). The number is read as $0100, which is 256 in hex. The next score is $0200, or 512 in hex.

This probably works ok in your program because you actually load the data backwards, in hl, l holds the small byte ($00-$FF) and h holds the large byte ($0100-$FF00). You load the small byte into h, then the large byte into l. Normally you do it this way:

Code:
If you did "ld hl,(Scores+2)", you would find that the values are mixed up. It's mentioned briefly in 28 days here:
http://tutorials.eeems.ca/ASMin28Days/lesson/day05.html#ram[/code]

@chickendude

I just learned about big and little endian. I just changed my code and loaded l before h and it seems to be working fine now. Thanks.