Second Smallalk Project (revised) Solution

This project is about building an Assembly Language Interpreter (ALI) for a Simple Assembly Lan-guage (SAL). Fortunately for you SAL has a limited set of instructions, described below, which makes it easier to write the ALI. SAL programs are executed on a virtual (emulated) machine consisting of a memory, which stores program code and program data, an accumulator register, an additional register and a Program Counter (PC), which keeps track of the instruction being currently executed. Your ALI can execute SAL programs one line at a time (in a sort of debug mode) or at once until a halt instruc-tion is encountered. Also, your ALI will include a graphical user interface that displays various kinds of information:




An editor eld that shows the state of computer memory in symbolic format (as opposed to binary or hex format).



An action button for executing one line of code, starting from the rst line of memory, numbered Line 0. See also the description of the Program Counter (PC) below.



An action button for executing the entire program, starting from the current value of the PC.



A set of widgets for displaying the content of the accumulator, additional register and PC. These widgets are updated each time one of the two action buttons above are selected.



The computer hardware uses 30-bit words and consists of the following components:




Memory. A 30-bit, word-addressable memory (RAM) for data, holding 256 words. Words are addressed by their location, starting from location 0 all the way up to location 255. Each location may either hold a signed integer in 2’s complement notation or a SAL instruction.



Accumulator. A 30-bit register. It is also known as Register A or A for short.



Additional register. A 30-bit register also known as Register B or B for short.



Program counter (PC). An 8-bit program counter (PC). The PC holds the address (number in program memory) of the next instruction to be executed. Before the program starts execution, the PC holds the value 0. It is subsequently updated as each instruction is executed.



A zero-result bit. This bit is set if the last ADD instruction produced a zero result. This bit is cleared if the last ADD instruction produced a result di erent from zero. The initial value is zero. The bit is changed only after ADD instructions are executed.



An over ow bit. This bit is set whenever an ADD instruction produces an over ow (i.e., a result that cannot be stored in 2’s complement notation with 30 bits). It is cleared if the ADD instruction did not produce an over ow. The initial value is zero.












The registers are used to hold data in arithmetic operations (i.e., additions). The program counter holds the index value (starting at 0) of the next instruction to be executed. SAL has the instruction set shown in Table 1.






DEC symbol
Declares a symbolic variable consisting of a single letter (e.g., X ). The




variable is stored at the memory location of this instruction.








LDA symbol
Loads byte at data memory address of symbol into the accumulator.








LDB symbol
Loads byte at data memory address symbol into B.








LDI value
Loads the integer value into the accumulator register. The value could be negative




but must be in the range of 30-bit 2’s complement numbers.








ST symbol
Stores content of accumulator into data memory at address of symbol.








XCH
Exchanges the content registers A and B.








JMP number
Transfers control to instruction at address number in program memory.








JZS number
Transfers control to instruction at address number if the zero-result bit is set.








JVS number
Transfers control to instruction at address number if the over ow bit is set.








ADD
Adds the content of registers A and B. The sum is stored




in A. The over ow and zero-result bits are set or cleared as needed.








HLT
Terminates program execution.










Table 1: Instruction set of SAL.



You may assume that SAL programs are entered correctly by users of ALI. (You are not required to perform error diagnosis or correction). Information in data memory and the registers can be displayed in binary, decimal, or hexadecimal format. Program source code should be displayed in symbolic (i.e., textual) format. Finally, you must use inheritance in your implementation. One good place for inheritance is to de ne an abstract superclass for SAL instructions with concrete subclasses for each particular instruction. Beware of in nite loops in SAL. Note that instances of the Smalltalk class SmallInteger use a 30-bit 2’s complement notation.




Extra credit. If you implement correctly the Command pattern from the Gang-of-Four book, you will receive a 20% increase on your project grade, up to 20 extra points.




You must work alone on this project. Your project code should be in a special package called CS474. Save all your code by ling out that package in the le xxx.st, where xxx denotes your last name. Submit the le by clicking on the link provided with this assignment. No late submissions will be accepted.