Help: ZX81 BASIC "Peek" function

Question

I need a way to find if the character ('<') has hit a wall (Black pixel Graphic)

-On a ZX81 game.

I'm been looking at another game... which uses code

if peek(peek 16398 +256*peek 16399) = code "**blackpixel graphic**" then ...

Which seems to work for them...

Is this correct code?

I'm not really knowledgable with addresses and getting memory and stuff.

Please help me...

-If you know a better way. Please answer :)

Thanks,

Trefoil

No help from me; I flushed that info from my memory about 20 years ago. :-) – Mercuric

xD :P Yeah I want to know the basics to pre-modern computing :) -Laying foundations... – Trefoil

"I want to know the basics to pre-modern computing " Then Try ZX-Spectrum instead. Much more fun + a good reason to learn a bit of Z80 assembly. you could try to make another driller(en.wikipedia.org/wiki/Driller_(video_game)), hard driving(en.wikipedia.org/wiki/Hard_driving) or another Elite(en.wikipedia.org/wiki/Elite_(video_game)) on spectrum. With text only display you'll be stuck with making rogue-likes(en.wikipedia.org/wiki/Roguelike) at best. – Misfeasor

I've already got a ZX81 now xD *I'll see how I get on :) – Trefoil

Perhaps this helps (if it is still of interest in 2021): Mastering Machine Code on Your ZX81. – Projectile

Answer 1

peek reads the byte at a memory location. According to The System Variables of the Sinclair ZX81, memory location 16398 and 16399 form a 16bit value containing the current "Address of PRINT position in display file".

Thus, peek 16398 + 256*peek 16399 combines the two values into a 16 bit address and peeking that address (possibly) gets the pixel/character(?) at that position.

Counterirritant

Yeah i've been looking at that. No help to a beginner though. =/ – Trefoil

Answer 2

Located at addresses 16398 and 16399 are two bytes that form the cursor location. (See http://web.ukonline.co.uk/sinclair.zx81/chap28.html). In other words,

peek 16398 + 256*peek 16399

gives you the memory address of the character on the screen where the next PRINT would go. Which apparently can be changed with PRINT AT.

peek(peek 16398 + 256*peek 16399)

finds the code for whatever character is at that location. The rest you should be able to figure out.

Now, the main question is: does your game use the cursor in the same way? If not you have to use a different solution.

Chloropicrin

Hmmm ok so if I print a black pixel graphic at 10,10 (or any position) 'peek(peek 16398 + 256*peek 16399)' will find the code for a black pixel graphic? – Trefoil

No, it will find the code for whatever graphic is at the current cursor position. – Chloropicrin

Cursor position? Huh? :P I never knew ZX81's had mouses or cursors? – Trefoil

Answer 3

It depends on what the memory address is, but peek usually means "what value is in this memory location?"

This looks like it should be some good reading in this particular topic.

Ambulacrum