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Object Oriented Programming Languages and
Environments

Second Smallalk project (revised)

Total points: 120
Full credit: 100 points (20 points extra credit)
This project is about building an Assembly Language Interpreter (ALI) for a Simple Assembly Language (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 instruction is encountered. Also, your ALI will include a graphical user interface that displays various kinds of
information:
1. An editor field that shows the state of computer memory in symbolic format (as opposed to binary
or hex format).
2. An action button for executing one line of code, starting from the first line of memory, numbered
Line 0. See also the description of the Program Counter (PC) below.
3. An action button for executing the entire program, starting from the current value of the PC.
4. 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:
1. 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.
2. Accumulator. A 30-bit register. It is also known as Register A or A for short.
3. Additional register. A 30-bit register also known as Register B or B for short.
4. 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.
5. 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 different from zero. The initial value is zero.
The bit is changed only after ADD instructions are executed.
6. An overflow bit. This bit is set whenever an ADD instruction produces an overflow (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 overflow. 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 overflow bit is set.
ADD Adds the content of registers A and B. The sum is stored
in A. The overflow 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 define an abstract superclass for SAL instructions with concrete subclasses for each particular
instruction. Beware of infinite 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 filing out that package in the file xxx.st, where xxx denotes your last
name. Submit the file by clicking on the link provided with this assignment. No late submissions will be
accepted.

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