CPU Sim

CPU Sim is a software development environment for the simulation of simple computers. It was developed by Dale Skrien to help students understand computer architectures. With this application the user is able to simulate new or existing simple CPUs. Users can create new virtual CPUs with custom machine language instructions, which are implemented by a sequence of micro instructions. CPU Sim allows the user to edit and run assembly language programs for the CPU being simulated.

CPU Sim has been programmed using the Java Swing package. This means that it is platform independent (runs on every platform that has a Java virtual machine installed).

Wombat 1 Sample CPU

A sample computer system, the Wombat 1, is provided with CPU Sim. It has the following registers:

pc (program counter); acc (accumulator); ir (instruction register); mar (memory address register); mdr (memory data register); status.

The assembly language of the Wombat 1 computer consists of 12 instructions. Each instruction is stored on 16 bits; the first 4 are the opcode and the other 12 are the address field.

Mnemonic Operation code Field length Meaningstop016stops the program executionload14 12transfers data from memory to the accumulatorstore24 12transfers data from the accumulator to memoryread34 (12)puts the data from the IO console to the accumulatorwrite44 (12)sends the data from the accumulator to the IO consoleadd54 12adds the data from memory to the accumulator and the result is then stored in the accumulatorsubtract64 12subtracts the data from memory from the accumulator and the result is then stored in the accumulatormultiply74 12multiplies the data from the memory by the accumulator and the result is then stored in the accumulatordivide84 12divides the data from the memory into the accumulator and the result is then stored in the accumulatorjmpz94 12jump to address if the accumulator is 0jmpnA4 12jump to address if the accumulator is negativejumpB4 12unconditioned jump to address

Features

CPU Sim has the following features:

allows the creation of a CPU (a virtual one), including the registers, RAM, microinstructions, and machine instructions; allows the creation, editing, and execution of assembly language programs for the simulated CPU; allows stepping forward and backward through the execution of assembly language programs.

Example program for the Wombat 1 CPU

This program reads in integers until a negative integer is read. It then outputs the sum of all the positive integers. Start: read // read n -> acc jmpn Done // jump to Done if acc < 0. add sum // add sum to the acc store sum // store the new sum jump Start // go back & read in next number Done: load sum // load the final sum write // write the final sum stop // stop

sum: .data 2 0 // 2-byte location where sum is stored

The following modification of the program is also used sometimes: Start: read // read n -> acc jmpz Done // jump to Done if nacc is 0. add sum // add sum to the acc store sum // store the new sum jump Start // go back & read in next number Done: load sum // load the final sum write // write the final sum stop // stop

sum: .data 2 0 // 2-byte location where sum is stored This one can use negative input to subtract, or 0 to break the loop.

CPU Sim

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