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.
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.
0 = Always 0
1 = Always 1
2 = RAM Shift Out from the ALU
3 = Q Shift Out from the ALU
5 = Flag Latch
6 = ALU Carry Out
7 = ALU Sign