Update: After first publishing this article I read the notes in the O2 MAME driver and it indicated that there was some other unexpected behavior with these registers. I did some testing and have updated the article with the new findings.
I recently set up a development system for the Odyssey 2 so I could dig into some of the capabilities of the system. I am looking to improve the documentation I wrote back in the 90's to support the home-brew community and maybe improve the O2EM emulator. The first thing I took a look at is the X/Y position registers which seem to work differently then any of the current documentation describes.
The purpose of these registers is to get the current X and Y position of the electron beam that is drawing the graphics on the CRT. Knowing the position could be useful to do mid-screen graphics changes. The registers are read from the VDC chip at location $A5 for the X position and $A4 for the Y position.
There seems to be two ways to read these registers. First you can bring bit one of the VDC Control Register ($A0) to 1 (&H02) and then back to 0. This will latch in the current beam position and they can then be read and will not change until this bit is strobed again. Second you can bring bit one of the VDC Control Register (&HA0) to 1 at which point the registers will follow the beam position, but you cannot read the Y position until you have first read the X position. The game Blockout read the Y position like this, it reads the X registers, throws away the value and then reads the Y.
. Bit one in the VDC Status Register (&HA1) is supposed to indicate that the beam has been latched, but it seems to just follow the state of the latch bit in the control register, so wouldn't seem to have a purpose, but I will explain later what I think it is for.
I did same tests on the X register and could not get reliable results from reading it. Even if it was reliable the CPU just wouldn't be fast enough to read the registers for any meaningful purpose.
The Y register starts at 0 at the end of VBlank and increments to $F7 before the start of the next VBlank. It remains $F7 for the duration of VBlank before resetting back to 0.
According to the Intel datasheet on the 8244 VDC chip, there is a pin called STB that can also be used to latch the beam position. This pin is not used on the Odyssey 2, but I assume the intention of this pin was to support a light gun. This would also explain the need for bit one in the status register which indicates that the position has been latched, I assume this bit would also be set by the external pin.
A test program can be found here that demonstrates the relationship between the scanline counter and the Y Position Register.
https://github.com/danlb2000/Odyssey-2/tree/main/PositionRegiserTest
The program resets the counter to a specified value and then records the value of Y Position register when the timer IRQ happens. It also changes the screen color at that time to demonstrate how mid screen color changes work. During the vertical blank it displays the value of the Y Position on screen. Adjusting the value of TIMER_START will show different values on the screen. The program can also be re-configured to show the value of Y Position during VBlank.
As of this writing neither MAME nor O2EM give the same results for this test as the real hardware.
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