The microcomputer system carries out complex tasks of the game by performing a large number of simple tasks. Control of the system is the primary function of the Microprocessing Unit. This Unit causes the system to perform the desired operations by addressing the Program Memory for an instruction, reading that instruction, and then executing the simple task dictated by that instruction. Temporary storage of data necessary for the execution of future instructions, such as arithmetic operations, is stored into a Read/Write Memory.
4.2.1 Program Memory (Bottom Half of Schematic Sheet 2):
Program Memory consists of read-only memories (ROMs), permanently programmed by Atari to execute the Dominos game. This memory has the capability of producing 8 bits of data for each of 4096 combinations of ones and zeros on the 13 address inputs. In computer terminology, this is stated as a memory size of 4k x 8.
The Dominos PCB is designed so that for the Program Memory two types of ROM devices can be used, mask-programmed ROMs or electrically-programmed ROMs (designated PROMs). The mask-programmed devices are programmed during manufacture of the device itself, and they have a memory capacity that is double that of the PROMs. The PROMs are electrically-programmed after their manufacture, but prior to being installed on the PCB.
At the factory all PROMs, all ROMs, or a combination of these device types may be installed for the Program Memory on a Dominos PCB. A table on PCB schematic sheet 1 shows the possible versions of the PCB, and lists device locations and device part numbers.
Since the data in the Program Memory is a permanent physical configuration of the ROM chips, the data is not lost when power is disconnected from the game or when the chip is removed from its socket. Since the Program consists of read-only memory, the result of an address input can only be the "reading" of data stored in the manufacturing process. It is not possible to "write" in more data.
4.2.2 Read/Write Memory (Top Half of Schematic Sheet 2):
ReadAVrite Memory (RAM) consists of random-access memory, which actually contains eight random-access memories (2102-1s). Data may be stored in the RAM (called "writing" the RAM), then later recalled (called "reading" the RAM). Memory size of the Read/Write Memory is 1k x 8.
In order to read from the RAM, the R/W (pins 3) input of all random-access memories must be high to write into the RAM, R/W must be low.
As previously mentioned, data stored in the RAM is for the purpose of performing operations on data as instructed by the Program Memory. Since the RAM is a temporary storage area, removing power from the RAM chips will "erase" all stored data.
4.2.3 Microprocessing Unit (Left Side of Schematic Sheet 3):
As mentioned earlier, the controller of the microcomputer is the microprocessor (MPU). From the MPU, a sixteen-bit address bus addresses Program Memory, RAM, and an Address Decoder. An eight-bit bidirectional data bus serves as a path for transferring data from Program Memory and to and from the RAM and other interfacing devices.
4.2.4 Tri-State Devices:
Tri-state devices, such as E5 on schematic-sheet 2, are capable of having normal logic output of ones and zeros when disable (pin 1) is low. When disable is high, the output becomes a high impedance. In other words, when disable is high, it is equivalent to completely removing device E5 from the circuit. ROMs and RAMs are also tri-state devices. Each ROM or RAM must be enabled by a certain logic level at its chip-enable input before the device is capable of outputting or inputting data.
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