Notes on signals - Useful links

BEAM computer Program Counter

The Program Counter (PC) of the BEAM computer presents few differences compared to the PC of Ben Eater’s SAP-1.

It is now an 8-bit register instead of a 4-bit one, therefore allowing 256 bytes to be addressed instead of just 16:

Schematic of the BEAM computer Program Counter

Schematic of the BEAM computer Program Counter.

The two Synchronous 4-Bit Binary Counters 74LS161 are connected in cascade according to the method illustrated on page 21 of the datasheet:

Cascaded counters

The Carry Out of a 4-bit binary counter, active upon reaching count 2^4, allows the next counter to increment its count by one unit. Two cascaded counters allow counting up to 2^4 * 2^4 = 16 * 16 = 256.

Notes on signals

  • The PC increment is performed by activating the PCI signal in the microcode (Program Counter Increment).
  • Loading the PC to a specific value following a jump instruction or return from subroutine is performed by activating the PCJ signal (Program Counter Jump). The Flags page includes a dedicated section covering jump operations in depth.
  • Ben Eater’s videos describing the operation of Flip-Flops and the construction of the PC.

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