Magnavox Odyssey 2

 

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The Odyssey 2, known as the Philips Videopac G7000 in Europe, was a cartridge based videogame console developed by Magnavox and released in 1978. My first exposure to this machine was back in the late 70's when one of my friends had one. In 1996 I wrote the first emulator of the system, O2EM. At the time there was no publicly available information on the system so I ended up having to reverse engineer it. Eventually the project was taken over by Andre de la Rocha who continued to update it. 

Processor: Intel 8048

Speed: .36 MHz clock speed

RAM: 128 Bytes + 64 Bytes in processor

ROM: BIOS: 1K (Internal to processor)
            Cartridges: 2K address space, 2 bank select inputs for up to 8K bank switched carts

Sound: Intel 8244 custom Audio/Video IC
             24-bit shift register, clockable at 2 frequencies; noise generator

Graphics: Intel 8244 custom Audio/Video IC

Graphics RAM: 256 bytes of video control registers
Colors: 16
Sprites: 4, 8x8 pixel, 1 color
Background Graphics: 9x8 grid of lines or blocks. Each segment individually controllable
          28 Character objects from predefined internal character set

I/O: 2, 8 direction digital joysticks with one fire button

        48 key QWERT membrane keyboard with reset button        

Ports: Cartridge ports doubles as a system expansion port

           Early versions of the system had joystick ports, but later systems hardwired the joysticks

The system was a big leap forward compared to the dedicated "pong" console Magnavox had been producing up to that point, but was not as flexible as the Atari 2600. The graphics system is quite limited with the four 8x8 sprites being the only objects where the programmer had complete control of the shape which made if difficult to port existing games to the system. The 8048, which was designed to be an embedded controller, is also a little tricky to use for game programming. It has a very lean instruction set, for example it has no subtraction instructions, and no comparison instruction. 

I have recently setup a development system that will allow me to run software directly on the hardware so I can study the operation of the system in more detail. This will allow me to update the technical documentation I wrote back in the 90s and possibly make some improvements to O2EM.

If you are interested in more information about the system I recommend this Facebook group which has a good collection of available O2 documentation and example code.

Odyssey 2 and Videopac Homebrew Games

Centurion FCC Shift and Carry

This is one of a series of articles about the Centurion FFC board's custom CPU. In this post I will look at the shift and carry logic. I will start with the shift logic. 

In the 2901 chip slice there are two places where data can be shifted, as it enters the RAM and as it enters the Q registers. On either end of the shift registers is a pin that can be used to shift a bit into the register or view the bit being shift out. Since the register can be shifted in either direction, these pins can behave as either inputs our outputs depending on the direction of the shift. 

These shift pins are cascaded between the two 2901s to create 8-bit shift registers with either ended labeled ALU.Q0/ALU.Q7 and ALU.RAM0/ALU.RAM1.

Looking at the FCC logic the 74LS241 is used to handle these bits depending on the direction of the shift. The enables on the chip are connected to ALU control bit 7 which will be low for down shifts and high for up shifts. Depending on the direction of the shift ALU.RAM0 or ALU.RAM7 is connected to Shift.RAMout and the other to Shift.RAMin. Q is connected the same way to Shift.QOut and Shift.Qin. 

Now lets look at where these signals are used. We will start with the ShiftIns. There are two 74LS151 multiplexers that handle the ShiftIns. The one for Q is controlled by microcode bits H10-H12 and the one for RAM by microcode bits H14-H16. 

The input options are the same for both registers:

0 = Always 0

1 = Always 1

2 = The shift out from the registers, thus forming a rotate operation

3 = The shift out of the other shift registers, this allowing Q to be shifted into RAM and RAM shifted in to Q.

4,5= The inverted and non-inverted Flag Bit. See my previous post for more details on these. 

6 = The Carry flag from the ALU

7 = The Sign flag from the ALU

The shift outs go a couple different places, most of which we have covered already. First, they go to Shift In Mux which I covered above, and they go to the Flag Mux which I showed in a previous post. The final place they go is to the Carry Mux which selects in the input to the ALU Carry In signal. The mux is controlled by the MP.H84-MP.H86 microcode bits. 

The input options for the Carry Mux are:

0 = Always 0

1 = Always 1

2 = RAM Shift Out from the ALU

3 = Q Shift Out from the ALU

4 = Inverted Flag Latch

5 = Flag Latch

6 = ALU Carry Out

7 = ALU Sign