Document Title: [Atari_xy.html (html file)]
The information in this document is not guaranteed to be accurate, use at your own risk. The Atari Analogue Vector Generator (or AVG) is the circuitry used to draw lines on the special XY monitors of some games from the late seventies and early eighties. These include Asteroids, Battlezone, Tempest, Gravitar and Star Wars, all of which used a slightly different AVG as the games evolved. The AVG is a small computer in its own right, it has its own ROM, RAM, instruction set and stack for subroutines. There are only a few instructions, which makes it quite easy to decode, and different games use similar ones even if they have different hardware (any takers for an AVG assembler / disassembler, maybe with GUI shape editing? ). The following information is for the Star Wars game, but is very similar to other games. The basic AVG instructions are as follows: 1. Draw vector relative X, Y. This instruction draws a vector from the last position to a new relative offset. 2. Centre. Returns beam to centre of display. 3. Set colour / intensity. Sets new colour and intensity for subsequent vectors. 4. Jump to new address. Jumps to specified address in vector ROM/RAM. 5. Jump to subroutine. Pushes current position to stack and jumps to specified subroutine in vector ROM/RAM. 6. Return from subroutine. Pops previous pushed address from stack. 7. Halt. Stops AVG until VGGO signal from main game processor. The main game processor fills the vector RAM with a list of instructions telling the AVG where to draw vectors on the display, and then writes to the VGGO address port which starts the AVG executing. The main processor can also reset the AVG by writing to the VGRST address. The AVG always starts executing instructions from address 0x0000 in the vector RAM after a reset or HALT instruction. The vector ROM probably holds the shapes and character set for the game as subroutines (I've never looked!) to be called from RAM when required. The character set is comprised of short vectors, so a special 'Draw Short Vector' instruction is implemented which uses less bytes per instruction. All vector instructions are two or four bytes long. The operand is packed with the opcode as the upper 3 bits. The instruction is stored in low byte-high byte format, as seen from the main processor. When calculating a vector jump address, note that the AVG sees the vector memory as 8K x 16 words, while the main processor sees 16K x 8 bytes. The draw instruction always draws relative to the end of the last vector with a negative number represented by the upper bit being set. There are 8 basic colors from the RGB bits and 256 intensity levels of these 8 colors. Further to this each vector draw instruction holds 8 levels of intensity which controls the overall intensity. I believe that this may have something to do with the fact that a long vector drawn in the same time period as a short vector will need to have more intensity to give the same apparent intensity on the screen. The scale instruction determines the size of subsequent vectors, so a shape can be magnified or shrunk by simply changing the scale factor without having to re-calculate all the vector points. The lower 8 bits of the scale instruction control a DAC which alters the analogue output swing of the X and Y DAC's. The upper 3 bits of the scale instruction are a digital scaling which I believe may shift the X and Y position data to achieve a binary multiplying effect on the vectors. I think a scale of 0x0FF gives a magnification of one. For subroutines there is a stack for holding return addresses. On earlier games the stack and instruction counter was built from discrete logic, but with Gravitar and Star Wars an 'Address Controller' gate array was introduced to replace this logic. With the discrete versions the stack was 4 deep, but with the gate array it could possibly be more. The last instruction after drawing all the required vectors must be a halt instruction, so that the main processor can poll the VGHALT bit and then update the new positions. Bit Key: X - X coordinate bits Y - Y coordinate bits I - Vector intensity(brightness) bits A - Vector address bits R - Red color bit G - Green color bit B - Blue color bit S - Scale (size) U - Unused bit For Star Wars the codes are as follows: Opcode Hex Binary Draw relative vector. 0x00 000YYYYY YYYYYYYY IIIXXXXX XXXXXXXX Halt 0x20 00100000 00000000 Draw short relative vector 0x40 010YYYYY IIIXXXXX New color/intensity 0x60 0110URGB IIIIIIII New scale 0x70 0111USSS SSSSSSSS Center 0x80 10000000 00000000 Jump to subroutine 0xA0 101AAAAA AAAAAAAA Return from subroutine 0xC0 11000000 00000000 Jump to new address 0xE0 111AAAAA AAAAAAAA I think that the instructions for Battle Zone, Gravitar and Tempest are the same as Star Wars, but older games are different. For Asteroids (and probably Asteroids deluxe and Lunar Lander): Draw relative vector 0x80 1000YYYY YYYYYYYY IIIIXXXX XXXXXXXX Draw relative vector and load scale 0x90 1001YYYY YYYYYYYY SSSSXXXX XXXXXXXX Beam to absolute screen position 0xA0 1010YYYY YYYYYYYY UUUUXXXX XXXXXXXX Halt 0xB0 1011UUUU UUUUUUUU Jump to subroutine 0xC0 1100AAAA AAAAAAAA Return from subroutine 0xD0 1101UUUU UUUUUUUU Jump to new address 0xE0 1110AAAA AAAAAAAA Short vector draw 0xF0 1111YYYY IIIIXXXX If you wish to see a constant image on the screen, remember that the main processor will have to poll the VGHALT until all the vector instructions have been executed, then write to VGGO to start again. Also make sure that you return the beam to the centre of the screen before halting the AVG, because if the beam were fully deflected and left there the monitor deflection power transistors may not like it!