• Memory Model:

Like all the calculators, memory is divided into cells consecutively indexed from 0 to 9 and six registers (R[0] - R[5]) with general application (used to store local variables and operations'intermediate results).

• Implemented Instructions:

What a calculator may do is defined by its Instruction-Set. An ISA efficiency is influenced by the instructions format.Every instruction contains the Opcode and other informations likeas where the operators are and where the results will be stored in. In thissimulation let's consider variable instructions lenghts and formats as follow: 

• Addressing Modes

Addressing Modes determinate how instructions select their operators (operator is a destination of a result too!). Theese modalities have a different way to do to reach registers or memory cells:

• Immediate: The instruction index part contains operatoritself, rather than informations on where the operator is. It is called Immediate because it's read directly from memory at the same time the instruction is read, so its immediately available. It doesn't request addictional memory references; but an immediate operator could be only a constant type and the values number is limited by the frame dimensions.   
• Direct: Indicates an operator by its memory address, it's used to reach variables stored into memory whom indexes are known at compilation time; instruction will always rech the same memory location.     
• Register: Register's addressing mode is the same of theDirect one; it indicates a register index rather than a memory address. Becauseof the importance of the registers, in most cases this addressing mode isused by all calculators.     
• Indirect: With this mode the specified operator is stored in memory or it's going to a memory location, but its address is in a register. When an index is used in this way it's called pointer; this addressing mode could reach a memory location without paying the price of having a full memory address into the instruction.
• Index: Operator is reached by a pointer in a register and an offset. This mode is used to refer memory from a memory pointer in a register and an offset into the instruction; or from an offset in a register and a pointer into the instruction.
• Based-Indexed: This addressing mode works like the Index one, but in this case offset and pointer are stored into two registers the first one is called index the second one is called base; so memory is addressed from two registers.

• Instructions detail:

• INC - INC(op) Increases by 1 specified operator. Op could be into memory or into a register.   
• ZER - ZER(op) set to 0 specified operator. Op could be into memoryor into a register.   
• LOAD - LOAD(dest, src) This instruction is used to load in a memory location, content of a register. "dest" is an in-memory operator, "src" is into a register.   
• STORE - STORE(src, dest) It's re reverse of Load, "src" is in a memory location, "dest" specifies a register.   
• MOVE - MOVE(dest, src) It's used to copy into dest (register)the content of src which could be a register or an immediate variable.   
• ADD - ADD(dest, op1, op2) Executes addiction between op1 and op2, and stores result into dest..   
• SUB - SUB(dest, op1, op2) Subtract op2 from op1and store result into dest.   
• MUL - MUL(dest, op1, op2) Multiplies op1 to op2 into dest.   
• MOD - MOD(dest, op1, op2) Stores into dest the rest of the division between op1 and op2.   
• AND - AND(dest, op1, op2) Stores into dest the And bit to bit operation between op1 and op2. 

click the little changer-man to start simulation

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