Memory is part of the CPU equipment. It is 4K x 12 bits, not extensible. Registers are implemented in the CPU equipment as well. The following diagram shows the CPU registers, the Memory, and how they connect to the Data (D-BUS) and Address (A-BUS) internal buses.
All registers are 12-bits.
The small triangles represent 3rd-state buffers isolating registers outputs from the buses. A typical register can output to both D-BUS and A-BUS; in the later case, the register is acting as a pointer to memory providing an "indirect address". Only one register can open to the same bus at the same time.
Register A acts as accumulator holding the result provided by the Arithmetic and Logical Unit (ALU), which is a combinational circuit capable of performing 16 different operations including 2's complement and BCD arithmetic. The default operation is "pass-through", in which case the accumulator functions as a general purpose register.
Auxiliary (general purpose) registers B, C and D are mostly intended to serve as pointers to memory although they can be used to hold temporary data as well.
PC, SP, IR and F are the Program Counter, Stack Pointer, Instruction register and Flags register, respectively.
Register K (read-only) is a direct input from the Console Entry Switches.
Register S holds the current channel number and the current memory page as explained later. In the diagram, S:7-11 contributes a "page number" to a 12-bits direct address, being the other 7 bits supplied by the instruction's code (IR:0-6). This is explained later in section "Paged Memory".
