Machines with Memory - Models of Computation: Exploring the Power of Computing - page 138

3.10.1 The Register Set
A CPU is a sequential circuit that repeatedly reads and executes an instruction from its memory
in what is known as the fetch-and-execute cycle. (See Sections 3.4 and 3.10.2 .) A machine-
language program is a set of instructions drawn from the instruction set of the CPU. In our
simple CPU each instruction consists of two parts, an opcode and an address ,asshown
schematically below.

1 4 5 16

Opcode Address

Since our computer has eleven instructions, we use a 4-bit opcode, a length sufficient to
represent all of them. Twelve bits remain in the 16-bit word, providing addresses for 4,096
16-bit words in a random-access memory.
We let our CPU have eight special registers: the 16-bit accumulator (AC), the 12-bit
program counter (PC), the 4-bit opcode register (OPC), the 12-bit memory address register
(MAR), the 16-bit memory data register (MDR), the 16-bit input register (INR), the 16-
bit output register (denoted OUTR), and the halt register (HLT). These registers are shown
schematically together with the random-access memory in Fig. 3.31 .
The program counter PC contains the address from which the next instruction will be
fetched. Normally this is the address following the address of the current instruction. However,
if some condition is true, such as that the contents of the accumulator AC are zero, the program
might place a new address in the PC and jump to this new address. The memory address
register MAR contains the address used by the random-access memory to fetch a word. The
memory data register MDR contains the word fetched from the memory. The halt register
HLT contains the value 0 if the CPU is halted and otherwise contains 1.

1 1

HLT

Random-Access
Memory Unit

4096 16-bit words

1 12 1 12

PC MAR

1 16 1 16

OUTR MDR

1 16

1 4

AC

OPC

1 16

INR

ALU

Figure 3.31 Basic registers of the simple CPU and the paths connecting them. Also shown
is the arithmetic logic unit (ALU) containing circuits for AND , addition, shifting, and Boolean
complement.