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Essentially, the cartridge header contains all the information the ColecoVision BIOS need to boot up and run your game properly. Regardless on the functionalities you are willing to use from the BIOS, and your intention to use your own RST commands, this information is essential to know if you want to code a ColecoVision game, even more if you code in assembly codes.
The names used in the video are the official ones from the ColecoVision programmers' manual.
The cartridge header is composed of :
$8000 : CARTRIDGE
game type = bytes AA 55 , display CV logo title screen
test type = bytes 55 AA , execute your game no delay
otherwise it is invalid and display "INSERT CARTRIDGE"
$8002 : LOCAL_SPR_TBL
it is the memory address where is your sprites table
for their coordinates, patterns and colors.
$8004 : SPRITE_ORDER
it is a table of usually maximum 32 bytes
that simply say in which order each sprite entry
should display. The corresponding initialisation function
set the values as 0,1,2,3,4... which mean that the first
entry (entry 0) is first, then the second entry (entry 1)
is second to be disaplayed. By reordering these numbers, you
affect the order of sprites, allowing to do flickering if needed.
$8006 : WORK_BUFFER
it is usually a big memory RAM space that can be used by
some BIOS functions to calculate graphics manipulations
like mobile and semi-mobile objects, tiles rotations, etc.
$8008 : CONTROLLER_MAP
it contains first 2 bytes telling if port-1 and port-2 are
enabled to be analyzed with the POLLER function. The following
values correspond to different fire buttons, stick direction,
keypad and spinner values.
$800A : START_GAME
it is the address of the starting point of you code, your main code entry.
$800C - $801B : RST_8H_RAM - RST_30H_RAM
Are usually jumps to sub-programs in the cartridge or to BIOS functions.
$801E : IRQ_INT_VECT
It should be a jump to a sub-routine saving all the PAIRs
(AF, BC, DE, HL, AF', BC' DE' and HL') and then jump to BIOS
to count spinner value $1F88 -or- something similar made by
the programmer. To avoid an interuption within an interuption
it is highly recommanded to use DI and EI opcodes to encapsulate
this part of the code. "IM 1" should be done as soon as possible
in game program, like the first thing at the main game entry point.
$8021 : NMI_INT_VECT
It is the crucial part of all ColecoVision games. If you are not
using the CV logo screen, you can directly code your sequence
of actions to do while a non maskable interrupt (NMI) occurs.
To be safe, you should save all the regsiter PAIRs before
executing anything in the NMI function.
And most important, you should read the video status
Otherwise there will be no other non maskable interupt.
$8024 : GAME_NAME
It is a string that contains first the name of the game
with usually the trademark symbol (characters 1E and 1F),
a slash as a separator, then the usual PRESENTS with the original licensor,
a slash separator again, and finally the release year in 4 digits.
That's all you really need to know.
Note for people who want to make optimized (small) games, take advantage of the ColecoVision title screen to initialize already the sound chip, graphic mode 1, and random number generator.
Edited by newcoleco, Thu Aug 26, 2010 8:10 PM.
