BASIC to ML: For Loop

The VZ200

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This page shows you how to make a Simple For Loop in Machine Language just like you would in BASIC.

Here is the For Loop in BASIC:

For x = (a + b) to a step -1

// Stuff

Next

In Assemble Language:

LD HC,(b-1)

LD BC,(a-1)

LD (mm),HL

ADD HL,BC

// Stuff

LD HL,(mm)

LD A,0

DEC HL

CP L

JP NZ, (START)

CP H

JP NZ, (START)

mm is two blank memory spots mm and mm+1 to store HL

HL is the same as X

START is the address of where Line 3 begins

The ML alternative only works for a + b is less then 65,535.

It is however much faster.

The basic framework should work for most of your For Loop needs.

Example:

To fill the screen with red squares

For x = 29183 to 28672 Step -1

Poke x, 191

Next X

8000: LD HC, 0200

8003: LD BC, 6FFF

8006: LD 7FFE, HL

8009: ADD HL, BC

800A: LD (HL), BF

800C: LD HL, 7FFF

800F: LD A, 0

8011: DEC HL

8012: CP L

8013: JP NZ, 8006

8016: CP H

8017: JP NZ, 8006

VZ200 is maintained by me Ben Pazolli in Perth, Western Australia. Please E-Mail me at bpazolli@gmail.com