Document Title: [AtariColorVecGen.html (html file)]
Last Modified on 3/14/96
The Atari vector generator is a state machine run that runs
independently of the 6502 processor. The state machine has its own
instruction set as well. Most of these instructions are two bytes long while one
is four. The VG can do many tasks with its time. It can: HALT itself and tell the
6502 that it is done, Move the beam to the CENTER of the screen, Load
SCALE info to the binary and linear scaling registers, Load STATS to the stat
registers, JMP to a new memory location, JSR to a new location saving the old
address on the stack, RTS from a Subroutine by pulling an address off of the
stack, and two DRAW commands; SDRAW(short vector) and LDRAW (long vector).
This information is for Tempest, other games all vary in minor way but have the
same general priciple.
The Vector Generator addresses its instructions in a peculiar way. Not in
order as most processors. The first address of any instruction is on the odd
byte and then back to the even byte. Therefore it is always addressing odd to
even. Example:
Data Listing: $4000 00
$4001 45
$4002 25
$4003 40
$4004 .........
The order of execution is : 4001,4000,4003,4002,4005,4004.......
This is important so keep it in mind when your looking at a vector generator
listing!!!!!
The format for the instructions are:
OOOYYYYY Word 1
ZZZXXXXX Word 2
Where OOO is the opcode, ZZZ is the relative intensity, YYYYYY and
XXXXX are data.
SDRAW: opcode 010, 2 bytes
Format: OOOYyyyy
ZZZXxxxx
The DRAW opcode(010) will be discussed first. yyyy is the
length in the y direction, and xxxx is the lenght in the x direction. Any
vector drawn is from the previous location of the vector. The most significant
digit of the lenghts is the sign bit. Zero is positive and One is negative, for
negative vectors the lenght is the one's complement of yyyy or xxxx.
Here are some examples:
This following procedure draws a capital letter T
Hex string: 20 4C 2C 40 1A 40 20 54
Order of Execution: 4C OPC=010 YYYYY=0C
20 ZZZ=000 XXXXX=00 Beam off
40 OPC=010 YYYYY=00
2C ZZZ=001 XXXXX=0C Beam on
40 OPC=010 YYYYY=00
1A ZZZ=000 XXXXX=1A (-6)
54 OPC=010 YYYYY=14 (-12)
20 ZZZ=001 XXXXX=00
Hopefully that clears things up a bit on the drawing of the vectors. The
LDRAW command is the same with the addition of two more data bytes.
Its format is as follows:
LDRAW: opcode 000, 4 bytes
OOOYYYYY Word 1
YYYYYYYY Word 2
ZZZXXXXX Word 3
XXXXXXXX Word 4
The added bits of data allow the vectors that are drawn with this
command to be very long or very precise( if they are scaled down ). NOTE:
on the LDRAW command a vectors actual lenght is the YYYYYYYYYYYYY lenght
divided by two!!!
The Remaining instructions follow:
HALT: opcode=001, bytes 2
001***** Word 1
******** Word 2 *don't care
SCALE: opcode 0111, bytes 2
011,1,*BBB Word 1
LLLLLLLL Word 2
Where: BBB is the Binary Scaling factor. Default=001
LLLLLLLL is the Linear Scaling Factor
STAT: opcode 0110, bytes 2
011,1,0*** Word 1
ZZZZ**** Word 2
Where: ZZZZ is an alternate intensity for the vector
011,1,1*** Word 1
****CCCC Word 2
Where: CCCC is a color number.
CENTER: opcode=100 , bytes 2
100***** Word 1
******** Word 2
JSR: opcode=101, bytes 2
101AAAAA Word 1
AAAAAAAA Word 2 Jump Sub A
RTS: opcode=110, bytes 2
110***** Word 1
******** Word 2
JUMP: opcode=111, bytes 2
111AAAAA Word 1
AAAAAAAA Word 2 JUMP A
As with the old VG, the newer one is also dual ported. This means that the
6502 and the VG can access the VG RAM/ROM at at the same time. The
addresses are identical which is different from the old system but more
convenient. The reason for this is because on the JUMP and JSR instructions
the 13 bit of data are left shifted one position and a 1 is brought in, giving
you the effective address.
EXAMPLE: F8 OPC=111 AAAAA=18
58 AAAAAAAA=58 A=1858
Left shift 1858 and bring in a 1 to the low bit
1858<1 = 40B1 Effective Address
Alpha/numerics Coding:
I know that this is the format for Major Havoc, but I haven't
checked the validity on any other games yet. I assume that they are
all the same though! First a little background.
The Alpha/Numerics are stored in a lookup table in the vector roms.
The table contains calls to the appropriate drawing subroutine which is
a two byte instruction (JSR instruction). The order of the table and it's
contents are shown during the crosshatch/alpha/numerics screen of the
self test, so you can see for yourself. [Space]=00 0=02 1=04 2=06 ect....
after 9 is A and after Z are various puctuations marks(these don't exist
in older games. If the letter is the last in its string the high order
byte of it's code is set to one making the code negative, this way the game
doesn't need any lenghth data for each string.
EXAMPLE:
00 02 04 86 Prints-> 012 Notice the code for "2" is now 86
