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  • Programming the G7000 / Odyssey² BIOS
  • Demo programs for the G7000 / Odyssey²
  • RAM cart for the G7000 / Odyssey²
  • KTAA, my first game for the G7000 / Odyssey²
  • Cart PCBs for the G7000 / Odyssey²
  • The MegaCART for the G7000 / Odyssey²
  • The FlashCART for the G7000 / Odyssey²
  • The VPP graphics editor for Videopac G7400+ and FlashCART
• BIOS
  • Introduction
  • First example: "HELLO WORLD"
  • Tables: Using interrupts to change VDC registers
  • A more complex example: joystick, sprites
  • How to check for collisions
  • How to play the built-in tunes
  • The clock routines
  • Bit fields and keyboard
  • Interrupts: VSYNC and line
  • Bank switching
  • Other useful routines
  • Custom sound player
  • Differences between PAL and NTSC machines
  • The Videopac+ G7400
  • Tips and tricks
  • About the hardware
  • Summary

How to check for collisions

This program demonstrates how to do collision checks between objects on the screen. To have something to collide with I put two horizontal grid lines, one vertical grid line, one ship shaped character and sprite 1 shaped as a triangle on the screen. To the right I display the contents of the collision register. It shows which objects collide with sprite 0 which is the joystick controlled ball. The joystick and sprite move routines are recycled from the previous chapter, they are not explained again. Since the part of the program which displays the static objects mostly moves values into the correct VDC registers I don't explain it here.

Setting and reading the collision register

It is only legal to read the collision register during VBLANK. Fortunately the IRQ routine in the BIOS is doing this. The value is stored in internal RAM at 03Dh. But before I can check the value, I have to tell the VDC which object to test for collision. So at the beginning of every frame I write vdc_coll_spr0 = 001h into the register vdc_collision, which means I want to know what collides with sprite 0 in the frame currently drawn. Then I read the collision register stored in the internal RAM by the IRQ and display it as hexadecimal using a very simple routine not explained here. This value represents the situation as drawn in the last frame. If you move sprite 0 using the joystick you can see which bit of the collision register is responsible for which object. The external collision is used only on Videopac plus machines, there it flags a collision with an object drawn in bright colors. The demo program uses only normal Videopac graphics, so that feature is not demonstrated here.

        call    waitvsync       ; wait for begin of frame

        call    vdcenable       ; enable vdc

        ; tell the vdc to look for collisions with sprite 0
        ; in the next frame
        mov     r0,#vdc_collision
        mov     a,#vdc_coll_spr0
        movx    @r0,a           ; activate collision checks

        ; display the contents of the collision register on
        ; the right side. this information reflects the
        ; situation of the last frame.
        call    gfxoff
        mov     r0,#iram_collision
        mov     a,@r0
        mov     r0,#vdc_char0
        mov     r3,#040h        ; x position
        mov     r4,#01Eh        ; y position
        mov     r6,#col_chr_white
        call    displayhex
        call    gfxon

This is the first demo program which is longer than one page (256 bytes), so I put all code which uses data in program memory (accessed via movp) into an extra page, this includes the routine to set the sprite shape and the display a hex number routine. But this means there is some unused space at the end of page 4, in bigger projects it could be necessary to use this space. The only way to solve this is to shift routines internally until everything fits and every movp gets its data from the correct page.