Vectrex tutorial II – General Tips

General Tips

CMP/FLAGS/Branches

A construct like

   bne overstep    
bra somewhere
overstep

can be optimized to

    beq somewhere
overstep

CMP

    lda value
    cmpa #0
    beq somwhere

is the same as:

   lda value
   beq somewhere

   lda value
   cmpa #0
   blt somwhere

is the same as:

   lda value
   bmi somewhere

   lda value
   cmpa #0
   bge somwhere

is the same as:

    lda value
    bpl somwhere

Further example:

    lda Joy2X
    cmpa #0
    blt left
    bgt right
    bra move_end
left:

is the same as:

    lda Joy2X
    beq move_end
    bpl right
; left is „fall thru"
left:

Y / X!

Due to our mathematical background we LIKE to order coordinates like x, y in RAM that would be like:

 bss
 org $c880
 xpos ds 1
 ypos ds 1

but Vectrex expects always first y position THAN x positions, a definition like:

 bss
 org $c880
 ypos ds 1
 xpos ds 1

makes much more sense, since we than can load and transfer a set of coordinates very often in register “D” – like:

 ldd ypos

Loops

Also loops, “naturally” if we would like a loop from 0 to 5 we start from 0 and compare some variable if we reached 5.
In assembler that is not very performant, compare those two variants:

    bss 
    org $c880
counter ds 1   

code

    lda #0
    sta counter
loopstart
    ... do something
    inc counter
    lda counter
    cmpa #5
    blo loopstart
    ... loop done

with

    bss 
    org $c880
counter ds 1

    code
    lda #5
    sta counter
loopstart
    ... do something
    dec counter
    bne loopstart
    ... loop done

Struct

Compare these two codes and tell me what is better readable (and which you could explain 2 years after you coded it):

; move bullet
    ldd  2,x ; add dx to x
    addd 6,x
    std  2,x
    ldd  8,x ; dy affected by gravity
    subd #3
    std  8,x
    addd 4,x ; add dy to y
    std  4,x
    ...

with

; move bullet
    ldd  B_XPOS,x ; add dx to x
    addd B_DX,x
    std  B_XPOS,x
    ldd  B_DY,x ; dy affected by gravity
    subd #GRAVITY
    std  B_DY,x
    addd B_YPOS,x ; add dy to y
    std  B_YPOS,x
    ...

The easiest way to do that is with the usage of a structure, like:

 struct BULLET
    ds B_DAMAGE, 2
    ds B_XPOS, 2
    ds B_YPOS, 2
    ds B_DX, 2
    ds B_DY, 2
 end struct

Which defines all offsets “automatically” (also if you want to insert new variables – you can do that without changing ANY code).

Optimizations

LDB memoryLocation
is 2 cycles faster than
TST memoryLocation
If you have a free register, when you want to test something out – just load it!

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