A stack machine is a simple implementation model for executing computer programs. In fact, in some development environments, there are compilers that translate a high-level computer language into a set of instructions to be executed using the stack machine model (e.g., the Java virtual machine instruction set). A stack machine consists of a stack, instructions, operands, a symbol table, and an evaluation model.
Assume that we have a stack machine that supports the following instructions from Hilderman's Algorithm Language (known as HAL) and the associated evaluation criteria for each instruction:
a.declare x: Inserts an integer variable named x in the symbol table and initializes it to zero (e.g., declare length). A variable must be declared before it can be used in an instruction. A valid variable name may consist of letters and numbers, but it must start with a letter.
b.read: Pushes a literal integer value read from the keyboard onto the stack (e.g., read).
c.push a: Pushes the literal integer value a onto the stack (e.g., put 8).
d.get x: Locates the variable x in the symbol table and pushes its value onto the stack (e.g., get length).
e.add: Pops the top two values off the stack, adds them, and pushes the result back onto the stack (e.g., add).
f.multiply: Pops the top two values off the stack, multiplies them, and pushes the result back onto the stack (e.g., multiply).
g.subtract: Pops the top two values off the stack, subtracts the second operand from the first, and pushes the result back onto the stack (e.g., subract).
h.divide: Pops the top two values off the stack, divides the first operand by the second, and pushes the result back onto the stack (e.g., divide).
i.set x: Pops the top value off the stack and assigns it to the variable x in the symbol table.
j.write: Pops the top value off the stack and writes it to the screen at the current cursor position (e.g., write).
k.writestring s: Writes the string s to the screen at the current cursor position (e.g., writestring The volume of the box is:).
l.writenl: Writes a new line character to the screen and positions the cursor at the beginning of the next line (e.g., writenl).
m.compare: Pops the top two values off the stack and compares them (e.g., compare). The result of the comparison is stored in a special integer variable named status. Specifically, if the two values are equal, it assigns 1 to status. Otherwise, it assigns 0.
n.jumpequal a: If the value of status is 1, it assigns the value -1 to status and branches to the instruction indicated by the literal integer value a (e.g., jumpequal 3). Otherwise, it just assigns the value -1 to status.
o.jump a: Unconditionally branches to the instruction indicated by the literal integer value a (e.g., jump 3).
p.end: Stops the program.
As you can see from the instructions described above, the operands are limited to variable names and literal integer values (except for writestring). All arithmetic operations are integer operations. Together, all of the instructions and the associated evaluation criteria describe the evaluation model.
Implement a program that uses a stack machine to interpret a program written in the HAL instructions given above. The input to your interpreter program will be a file containing a HAL program. Your interpreter program should prompt the user for the name of the file containing the HAL program. You can assume the file contains a complete and correct HAL program. That is, you don't need to worry about handling syntax errors. An example HAL program is shown below. Here, each line of the program is associated with a unique, sequential integer (i.e., the line number), where the first line number is 0.
0 declare a
1 declare b
2 declare c
3 push 3
4 set b
5 get a
6 read
7 add
8 set a
9 push 1
10 get b
11 subtract
12 set b
13 get b
14 get c
15 compare
16 jumpequal 18
17 jump 5
18 writestring the sum is:
19 get a
20 write
21 writenl
22 end
Before interpreting a HAL program, the instructions in the file should be read into a program array implemented as an unsorted array-based list defined as a C++ class, where each element consists of a struct containing two members: (1) a string operation member and (2) a char* operand member. For example, the following program array contains the example HAL program shown above. Here, each line of the program is associated with a unique, sequential integer (i.e., the array index doubles as the line number, so the line number from the input file does not need to be stored in the program array). A special variable of type int named pc (i.e., program counter) should be used to indicate the next instruction to execute. see image.
A HAL program always starts executing at line number 0. Prior to executing an instruction, your interpreter should print the line number to the screen. For example, when interpreting the sample HAL program given above, your interpreter would print
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 5 6 7 8 9 ...
meanwhile, a miracle happens ... 22
The symbol table should be implemented as a sorted array-based list defined as a C++ class, where each element consists of a struct containing two members: (1) a string symbol member and (2) an int literal integer value member. The following array contains the symbol table generated by interpreting the first three lines of the sample HAL program.
| a | 0 |
| b | 0 |
| c | 0 |
| ... | ... |
The stack should be implemented using as an array-based stack defined as a C++ class, where each element contains only literal integer values. Values are pushed onto the stack and popped off the stack according to the execution model. The following array contains the stack generated by interpreting the first seven lines of the sample HAL program (this assumes the value read from the keyboard was 10).
| ... |
| 10 |
| 0 |
Your C++ classes should be implementations of the algorithms given on the CS210 Algorithms web page. Do not waste your time implementing any algorithms that you don't actually need.
The program usefulCodeMaybe.cpp from the directory /home/venus/hilder/cs210/assignment4/datafiles should give you some idea as to how your program array might be used. Note that this program is not "the answer".
For programming problems, the Results are worth 70%. So, for this problem, the Results are worth 70 marks out of the 100 marks available. Demonstrate that your interpreter works by interpreting a HAL program that you have written yourself. Your HAL program will prompt the user to enter a test score from the keyboard and then read the test score from the keyboard. If the test score is not -1, your HAL program must add the test score to the total score and prompt for more input. If the test score is -1, your HAL program must stop prompting for input, print the total score and the number of test scores entered, calculate the average of the test scores, write the average to the screen, and terminate. Your demonstration should be captured in script files.
Use the following three sequences of test scores as input to your HAL program:
Test 1: 77 86 54 95 62 100 81 73 39 88 52 67 45
Test 2: 155 201 318 184 279 126
Test 3: 3 9 14 2 8 11 5 10