Simple CPU

A simple CPU has a system clock, a logic control unit, an arithmetic logic unit, an Instruction Counter, an Instruction Register, an Address Register, and an Accumulator.

Instruction Fetch and Decode Cycles:

Execute and Store Cycles:

A directive selects options that affect translation of a program, or that tell system software what to do with the program while it is being translated or being loaded for execution.  Classification of an action as directive or instruction is language-dependent.  Computer Science majors learn more details about directives when studying compilers and assemblers. The term "pragma" is used with Ada to specify compiler options.

Memory locations in a computer are numbered.  The lowest memory location is location 0.  The computer references each location in memory by a number.  That number is called an address.  The bit pattern stored in memory at that location is called the contents of that location.  The order of numbering bits in a computer word, and of storing words in memory, is done by the design engineer.  

Registers often have more bits than main memory.  This accommodates additional precision during a sequence of computations.  Main memory usually does not have any ability to do anything with data except hold it and transfer it.  Registers also have special logic that main memory does not have.  For example, it is common to be able to increment registers, do bit manipulation in registers, and other activities.

The first sketch below shows the accumulator with Least Significant Bit (LSB) as bit 0, and the Most Significant Bit (MSB) as bit 15.  Conceptually bend the register into a circle to make it a ring.  With this is the mental model, you can rotate bits through the register.  For example, to rotate bits right 2 places, old bit 0 goes to new position 14; old bit 1 goes to new position 15; old bit 1 goes to new position 1; and so forth.

In the example below, the 4-digit hexadecimal numbers represent data and a short program loaded into memory before the program is executed.  The blank locations could contain 4-digit hexadecimal number.  The Instruction Counter is initialized to the entry point of the program prior to the beginning of execution.

The contents of other memory locations are arbitrary before execution begins.

The contents of the Instruction Counter is 0006.  In binary, the contents is 0000000000000110.

The only thing the memory knows about are the ones and zeros.  The machine does not know about hexadecimal, and it does not care.  We record the contents in hexadecimal to make it easier for us.

Below is a trace of the execution of the program listed above.  

The Instruction Counter is initialized to 0006 by specification of the Entry point prior to the program starting.

In the Execute phase at time 1, the contents of memory location 03 is added to the contents of the Accumulator.  What occurs is the addition of the decimal numbers 9 + 5 = 14, which is 0009 + 0005 = 000E in hexadecimal.

Real computers have many instructions.  Simple real computers have about 100 instructions.  Main frame computers may have on the order of 200 to 300 instructions.  The type and number of instructions are designed into a CPU to give achieve goals of performance and cost.

The Simple CPU for class use was constructed to have only 32 instructions.  The Simple Computer has 256 memory locations numbered 0 through 255.  The memory is word-addressable, 16 bits per word.

Limiting the instruction set to 32 instructions imposes severe limitations on the efficiency of the computer.  I wanted to illustrate basic input and output, load and store, simple arithmetic, simple register operations, simple conditional instructions, simple addressing and indirect addressing.  There may be better combinations of instructions that will yield a more efficient system.  I invite suggestions for use next semester, subject to the constraint of only 32 instructions.  After trying this instruction set on a few examples, better instruction mixes should become apparent.

Instructions in RED are ones used in "CPU Add" homework.

Subtraction is implemented by forming the two's complement by performing NOT, followed by INC.  Shifting and bit testing instructions are needed to implement multiplication.

Thoughtful assignment of machine codes to instructions would group families of instructions according to specific simple bit patterns.  Attention to this detail has not been given for this draft.

A symbolic address identifies a location in memory by using symbols rather than numbers.  When a symbol is used on the left side of an instruction, the symbol is defined as the number of the associated memory location.  In the below program, the symbolic address P is defined as memory location 02.  Symbolic address Q is defined as memory location 03.  Symbolic address Answer is defined as memory location 04.  When a symbolic address is used on the right side of an instruction, the number associated with that symbolic address is substituted.

ORG tells the loader where to begin loading the program into memory.
ENTRY tells the computer where to begin execution after code is in memory.  It accomplishes this by placing the argument into the Instruction Counter before execution begins.

Compared to the earlier trace, this one records entries using symbolic codes, symbolic addresses, and decimal numbers.  The first trace example more closely reflects what the computer actually does.