Finally this input reads from the Constant ROM. Unlike a normal CPU, the FFC's CPU cannot included data as part of an opcode so this is used to provide constant data to the CPU.
The is one of a series of articles about the Centurion FFC board's custom CPU. In previous posts I explained how the program is stored as microcode. In this post we will take a look at what each bit in he microcode instruction does. In later posts I will dig deeper into some of these functions.
The first set of signals control the 2901 Chip Slice which I discussed in an earlier post.
These signals select which 2901 register goes on the A and B inputs to the ALU.
MP.H50 - ALU A Select 0
MP.H51 - ALU A Select 1
MP.H52 - ALU A Select 2
MP.H53 - ALU A Select 3
MP.H54 - ALU B Select 0
MP.H55 - ALU B Select 1
MP.H56 - ALU B Select 2
MP.H57 - ALU B Select 3
These signals select the ALU source, function and destination.
MP.H60 - ALU Source Select 0
MP.H61 - ALU Source Select 1
MP.H62 - ALU Source Select 2
MP.H64 - ALU Function Select 0
MP.H65 - ALU Function Select 1
MP.H66 - ALU Function Select 2
MP.H70 - ALU Destination Select 0
MP.H71 - ALU Destination Select 1
MP.H72 - ALU Destination Select 2
The next set of signals selects the source for the bit that gets shifted into the 2901 Q register and RAM register during a shift operation. This bit could get shifted into either the high or low bit depending on the direction of the shift.
MP.H10 - Shift Qin Select 0
MP.H11 - Shift Qin Select 1
MP.H12 - Shift Qin Select 2
MP.H14 - Shift RAM In Select 0
MP.H15 - Shift RAM In Select 1
MP.H16 - Shift RAM In Select 2
These three lines select what will go into the carry input of the 2901's ALU which is used during addition and subtraction operations.
MP.H84 - ALU.Carry_IN Select 0
MP.H85 - ALU.Carry_IN Select 1
MP.H86 - ALU.Carry_IN Select 2
MP.H34 - Branch Select Invert
MP.H35 - Branch Select 0
MP.H36 - Branch Select 1
MP.H37 - Branch Select 2
MP.H30 - Flag Mux Invert
MP.H31 - Flag Mux 0
MP.H32 - Flag Mux 1
MP.H33 - Flag Mux 2
In my last two posts I took a look at the two key components that form the heart of the Centurion FFC board's custom CPU. Now let's take a step back and take a look at an overview of the board. The FFC is actually composed of two boards, one is primarily for the CPU and the other for the drive control, but there is some overlap.
This is the board that contains the drive control CPU which is the one I am going to focus on in these posts.
In my last post I described the AMD 2901 Chip Slice which forms the core the Centurion floppy disk controller. The 2901 provides the two fundamental components of a CPU, registers and an Arithmetic Logic Unit (ALU). The question is, how do you turn a 2901 into something that can execute code? The key to this is microcode.
You can think of microcode as machine language instructions, but each bit of the microcode instruction controls a single signal within the CPU. For example the the 2901 has eight inputs which control the ALU function, each of these inputs would be a single bit in the microcode instruction. It is not uncommon for microprocessors to use microcode under the hood, where it is hidden from the programmer. The programmer works with machine language opcodes each of which executes one or more microcode instructions. The Centurion FFC on the other hand uses microcode directly.
The Centurion FFC has seven microcode ROMs making for a 56 bit wide microcode instruction, and each is 1K. There is also an eighth ROM that store immediate data instead of control signals which can be fed into the 2901 and other parts of the CPU. Before we can look at what the microcode bits do, we need to see how microcode is addressed. For that, the board uses the Signetics 8X02A Control Store Sequencer. Here is the block diagram of that chip:
In the early days of computers, processors were build from discrete components starting with relays, vacuum tubes, transistors and then small scale integrated circuit chips. In the early 1970's the first microprocessors were introduced which made it easy to construct personal computers but they lacked the processing power needed for mini and mainframe computers. Companies like AMD provided a middle ground between these two options in the form of large scale integrated circuit chips that provided the basic building blocks of a processor. The key component of the AMD family was the 2901 chip slice.
I first encountered this chip when I was working on the MAME driver for that Atari arcade game I-Robot. I-Robot was the first arcade game to used solid filled 3d polygons. The 2901 was used as the core of a custom processor that did all the 3D calculations.
More recently I started watching the Usagi Electric YouTube channel where he has been working on a Centurion mini-computer. You can see the playlist of videos about this project here:
The main CPU as well as some of the controller cards in this system were also based on the 2901. I am going to take an in depth look at the FFC floppy controller board. You can find a lot of information about this board in this Github repository. We will start by looking at the 2901 Chip Slice Processor.
Here is the block diagram of he 2901.
The chip has two major parts, the RAM block which is used to create 16 internal CPU registers, and the Arithmetic Logic Unit (ALU) which does addition, subtraction and logical operations. The RAM and ALU are each 4 bits wide, but you can combine multiple chips to create as wide a data path as is needed. This is why it is referred to as a chip "slice".
Now let's look at the inputs of the chip to better understand how it works. First there is the A and B address inputs. These are both 4 bits and select which location in memory is put on the A and B bus. To be clear there is only one block of memory, so if A and B are the same, then they are putting the same data on both busses.
The next input is 3 bits for the source selection, this will select what goes into the two inputs of the ALU as follows:
Recently Josh over at RetroTV1 Tech purchased a C64 that the seller said was in working order. He wasn't able to get it working so he sent it to me to see if I could repair it. He has put up a video showing what he did to try to get the system to work.
The system was very clean and in good condition.
The schematics for the Atari CPS Super Salt Diagnostic Assembly shows a connector labeled "Error Display Interface for Future Use". There are no schematics for this board and I haven't been able to find much information on this. Presumably this is used to display error codes, especially when the system doesn't work enough to get a display on the screen.
Besides power the signals going to the display board are the four direction pins on joystick ports 1 and 2 which can server as inputs or outputs (these are labeled D0-D7), the motor control, command and data outputs from the SIO port, and OE which is just an inversion of the command signal.
There are no schematics for this board and I haven't been able to find much information about it, but I did find a few pictures of the board. Here is a picture of the board installed in the test assembly. It is composed of two parts, the main board which plugs into the test unit with a connector at the top middle, and a second smaller display board which plugs into a card edge connector on the side of the main board.
Here is the display board. There are three empty sockets in this picture labeled CR2-CR4. CR is the reference designator for a diode so I assume these held 7-segment LED numeric displays.