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Basic Controls

CPU is a Central Processing Unit computer simulator which allows you to enter and execute assembly/machine language code. It has 4 speeds of operation, indicated by the icons at the bottom left corner of the screen. These are, in order, Reset (double left arrow), Stop (square button), Play (single right arrow), and Next Step (double right arrow).

The simulator has two modes. The simulator can express assembly code instructions, memory addresses, and variable values as either symbolic (ASCII) or binary formats. You can switch between these modes using the switch in the bottom center of the screen. Instructions are carried out by routing them through the appropriate portion of the CPU, and the movement of instructions will be indicated by flashing color change as the signal moves through the processor when the unit is in run mode.

Assembly Language

The simulator understands 14 instructions:
Binary Opcode Meaning
Data Flow
00010100 LOD X (or #X) Load from Memory location X (or number X) to the accumulator
00000100 STO Y Store contents of accumulator in Memory location Y
Control
00001100 JMP P Jump to instruction P
00001101 JMZ P If Acc = 0, go to instruction number P, otherwise, go to next instruction.
00001110 NOP No operation, but go to next instruction.
00001111 HLT Halt execution, do nothing more.
Arithmetic-logic
00000000 ADD X (or #X) Add contents of memory location X (or #X) to Acc.
00000001 SUB X (or #X) Subtract contents of memory location X (or #X) to Acc.
00000010 MUL X (or #X) Multiply Acc. by contents of memory location X (or #X)
00000011 DIV X (or #X) Divide Acc. by contents of memory location X (or #X)
00001000 AND X (or #X) If contents of Acc. and memory location X (or number X) are both 0, put 1 into Acc., otherwise put 0 into Acc.
00001001 NOT If Acc. contains 0, set Acc. to 1, otherwise set to 0.
00001010 CPZ X (Compare zero) If X = 0, set Acc. to 1, otherwise set A to 0.
00001011 CPL X (Compare less) If X < 0, set Acc. to 1, otherwise set Acc. to 0.

Clicking Play will simulate the execution of all the assembly code in RAM, until the last line of code or a HLT instruction is encountered. Signals from travelling from one component of the CPU to another will be highlighted along the wires, and component displays will update as new addresses and values are loaded into them. Clicking Next Step will step through the execution one line at a time. Reset will reset the PC.



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© 2003 PWS Publishing Company, All Rights Reserved.