Z80 Assembler

[Muppz]

Afternoon folks,

I'm having a play with an old msx and some of the assembler source code examples for the video processor have me a bit confused which is not hard to do these days and Z80 was never my strongest language.

the bit that is confusing me is the meaning of the following (its a section from how to program the palettes)

defb 4*0, 4*0, 4*0

defb 4*3, 4*7, 4*3

I may well be being a numpty but I've never come across that notation before.

My best guess is its some sort of array concept...

Can anyone clue me in?

Cheers

Chris

 

[Hurray Banana]

Looks like a assembler specific statement ptobsbly filling 4 bytes with zero then doing the same with another eight. the next statement filling the next 4 bytes with 3 then the next 4 with 7 then the final 4 with 3 again.

Just a guess mind

 

[cmonkey]

defb = define bytes

Assuming the assembler you're using treats '*' as multiply then the above would be the same as :-

defb 0,0,0
defb 12,28,12

Seems an odd way of defining bytes to me but it may well have been the norm for the person who originally wrote the examples!

 

[Muppz]

cheers cmonkey,

I think I eventually figured it out after seeing another example with 4 palette sets and it makes even less sense, the * appears to be being used like a delimiter, the 4 seems to mean the palette number and the bit after the * was the value for the Red, Green, Blue components.

That must have been one crazy assembler the original coder used.

 

[Hurray Banana]

ade you're explanation makes sense, not done assembly for so long fotget conventions i ysed to use. i remember now that I used to define look up tables using similar formatting.

 

[cmonkey]

You're right there Muppz, it sure must have been a crazy assembler that the original coder used! I've never seen a Z80 assembler use '*' as a delimiter, and I've certainly used a fair share of Z80 assemblers in my time!

 

[Muppz]

My thinking it was a delimiter was wrong and you were right about the multiplier.

The 4 palette thing was a co-incidence in that he set each palette so palette 1 was dfb 1*2, 1*3,1*0 palette 2 was 2*0,2*5,2*1 and so on for palettes 3 & 4

I've found it's his coding style to just multiplies values up a lot rather than put in the actual value, so to clear 4 palettes he just loads up a counter like this LD B,64 * 4

I suppose it makes sense but without comments in the source its hard work.

I've spent to much time with high level languages, I need to go back to being 13 again.

Thanks for input chaps.

 

[cmonkey]

The instruction LD B,64*4 would probably fail during assembly as it would overflow register B because it's only an 8-bit register (valid values 0-255) and 64*4 = 256. LD BC,64*4 would be OK though.

In contrast to you I spend ALL my time in low level languages and haven't a clue how to program in modern day high-level languages! I need to grow up and move into the modern world!

 

[Muppz]

you are correct it was a typo, should have been 64 *3

At least with ASM you don't have to worry about tracking down errors due to compiler optimisations and their inherent problems like if you write to a location and read it back form the location and its not what you expected because it tried to be clever and put it into a register and just read it back from there instead of the memory location you wanted to read.