Difference between revisions of "Z80 Good Programming Practices"

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(Ix thing was painfully wrong, lut code wasn't the best still isn't.)
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= Programming Techniques =
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== Lookup table ==
 
== Lookup table ==
  
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= Source Code =
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Some advices to take into consideration:
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* split into various files in a logical way (header, main, subroutines, data) when it turns many pages long.
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* document every routine with input, output, destroyed registers and a short description when appropriate.
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* document well ugly and difficult parts of code
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= Related topics =
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* [http://www.unitedti.org/forum/index.php?showtopic=8461 Common Mistakes and Good techniques]

Latest revision as of 12:53, 17 November 2010

Programming Techniques

Lookup table

If you have a place in your code where a value is tested to choose between a lot of things, like subroutines or data, it can be a good idea to use lookup tables instead of a series of tests. It makes the code more readable, concise and extensible.

In terms optimisation though it should be used when the data is not sequentially ordered or when the objects being pointed to are not the same size. For example, using LUTs (Look Up Tables) to find a tile in a block of memory that is only tiles would both slower and cost more memory. Using LUTs to find a particular string would be quicker but would waste more memory than a linear search. Using LUTs as a jump table to different code blocks located through out a program would be faster and smaller compared to the alternative.

However, if there aren't many jumps and many of the values of a are sequential, it would be more efficient to do something like:

 ld a,(Number)
 or a
 jp z,A_is_0
 dec a
 jp z,A_is_1
 dec a
 jp z,A_is_2
 sub 2
 jp z,A_is_4
 dec a
 jp z,A_is_5
 

Examples:

WithoutWith
 ld a,(SpriteNumber)
 cp 0
 jp z,ChooseSprite0
 cp 1
 jp z,ChooseSprite1
 cp 2
 jp z,ChooseSprite2
 cp 3
 jp z,ChooseSprite3
 cp 4
 jp z,ChooseSprite4
 cp 5
 jp z,ChooseSprite5
...
ChooseSprite0
 ld hl,Sprite0
 jp DisplaySprite
ChooseSprite1
 ld hl,Sprite1
 jp DisplaySprite
ChooseSprite2
 ld hl,Sprite2
 jp DisplaySprite
ChooseSprite3
 ld hl,Sprite3
 jp DisplaySprite
ChooseSprite4
 ld hl,Sprite4
 jp DisplaySprite
ChooseSprite5
 ld hl,Sprite5
 jp DisplaySprite
...
DisplaySprite
 ld bc,(coordinates)
 call SpriteRoutine

 ld a,(SpriteNumber)
 add a,a   ; a*2 (limits SpriteNumber to 128) 
 ld h,0 
 ld l,a 
 ld de,SpriteAddressLUT
 add hl,de
 ld a,(hl)
 inc hl
 ld h,(hl)
 ld l,a
 ld bc,(coordinates)
 jp SpriteRoutine
...
SpriteAddressLUT
 .dw Sprite0
 .dw Sprite1
 .dw Sprite2
 .dw Sprite3
 .dw Sprite4
 .dw Sprite5
 

And this one :

WithoutWith
 ld a,(MenuChoice)
 cp 0
 jp z,Choice0
 cp 1
 jp z,Choice1
 cp 2
 jp z,Choice2
 cp 3
 jp z,Choice3
 cp 4
 jp z,Choice4
 cp 5
 jp z,Choice5
 cp 6
 jp z,Choice6
 cp 7
 jp z,Choice7
...
 
 ld a,(MenuChoice)
 add a,a ; a*2
 ld h,0
 ld l,a
 ld de,CodeBranchLUT
 add hl,de
 ld a,(hl)
 inc hl
 ld h,(hl)
 ld l,a
 jp (hl)
...
CodeBranchLUT:
 .dw Choice0
 .dw Choice1
 .dw Choice2
 .dw Choice3
 .dw Choice4
 .dw Choice5
 .dw Choice6
 .dw Choice7
 

Source Code

Some advices to take into consideration:

  • split into various files in a logical way (header, main, subroutines, data) when it turns many pages long.
  • document every routine with input, output, destroyed registers and a short description when appropriate.
  • document well ugly and difficult parts of code

Related topics