The Little Man Computer (LMC) is an instructional model of a computer, created by Dr. Stuart Madnick in 1965. The LMC is generally used to teach students, because it models a simple von Neumann architecture computer - which has all of the basic features of a modern computer. It can be programmed in machine (albeit usually in decimal) or assembly code.
For reference, here is some example code that takes numbers as input, and squares the result. Credit Wikipedia.
START LDA ZERO // Initialize for multiple program run
INP // User provided input
BRZ END // Branch to program END if input = 0
STA VALUE // Store input as VALUE
LOOP LDA RESULT // Load the RESULT
ADD VALUE // Add VALUE, the user provided input, to RESULT
STA RESULT // Store the new RESULT
LDA COUNT // Load the COUNT
ADD ONE // Add ONE to the COUNT
STA COUNT // Store the new COUNT
SUB VALUE // Subtract the user provided input VALUE from COUNT
BRZ ENDLOOP // If zero (VALUE has been added to RESULT by VALUE times), branch to ENDLOOP
BRA LOOP // Branch to LOOP to continue adding VALUE to RESULT
ENDLOOP LDA RESULT // Load RESULT
OUT // Output RESULT
BRA START // Branch to the START to initialize and get another input VALUE
END HLT // HALT - a zero was entered so done!
RESULT DAT // Computed result (defaults to 0)
COUNT DAT // Counter (defaults to 0)
ONE DAT 1 // Constant, value of 1
VALUE DAT // User provided input, the value to be squared (defaults to 0)
ZERO DAT // Constant, value of 0 (defaults to 0)
In the absence of any Windows desktop emulator, I created my own in .NET and here it is. View and download on GitHub.