Question

(Modifications to the Simple Compiler) Perform the following modifications to the Simple compiler. Some of these modifications may also require modifications to the Simpletron Simulator program written in Exercise 8.19 a. Allow the modulus operator (s) to be used in let statements. Simpletron Machine Language must be modified to include a modulus instruction. b. Allow exponentiation in a let statement using \(\wedge\) as the exponentiation operator. Simpletron Machine Language must be modified to include an exponentiation instruction. c. Allow the compiler to recognize uppercase and lowercase letters in Simple statements (e.g., 'A' is equivalent to 'a'). No modifications to the Simulator are required. d. Allow input statements to read values for multiple variables such as input \(x, y .\) No modifications to the Simpletron Simulator are required. [Page \(1055]\) e. Allow the compiler to output multiple values in a single print statement such as print a, \(b, c .\) No modifications to the Simpletron Simulator are required. f. Add syntax-checking capabilities to the compiler so error messages are output when syntax errors are encountered in a Simple program. No modifications to the Simpletron Simulator are required. g. Allow arrays of integers. No modifications to the Simpletron Simulator are required. h. Allow subroutines specified by the Simple commands gosub and return. Command gosub passes program control to a subroutine, and command return passes control back to the statement after the gosub. This is similar to a function call in \(\mathrm{C}++.\) The same subroutine can be called from many gosub commands distributed throughout a program. No modifications to the Simpletron Simulator are required. i. Allow repetition statements of the form for \(x=2\) to \(1 \theta\) step 2 simple statements next This for statement loops from 2 to 18 with an increment of \(2 .\) The next line marks the end of the body of the for. No modifications to the Simpletron Simulator are required. j. Allow repetition statements of the form for \(x=2\) to 10 simple statements next This for statement loops from 2 to 10 with a default increment of \(1 .\) No modifications to the Simpletron Simulator are required. k. Allow the compiler to process string input and output. This requires the Simpletron Simulator to be modified to process and store string values. [Hint: Each Simpletron word can be divided into two groups, each holding a two-digit integer. Each two-digit integer represents the ASCII decimal equivalent of a character. Add a machine-language instruction that will print a string beginning at a certain Simpletron memory location. The first half of the word at that location is a count of the number of characters in the string (i.e., the length of the string). Each succeeding half word contains one ASCII character expressed as two decimal digits. The machine-language instruction checks the length and prints the string by translating each two-digit number into its equivalent character. I. Allow the compiler to process floating-point values in addition to integers. The Simpletron Simulator must also be modified to process floating- point values. (A simple Interpreter) An interpreter is a program that reads a high-level language program statement, determines the operation to be performed by the statement and executes the operation immediately. The high-level language program is not converted into machine language first. Interpreters execute slowly because each statement encountered in the program must first be deciphered. If statements are contained in a loop, the statements are deciphered each time they are encountered in the loop. Early versions of the BASIC programming language were implemented as interpreters.

Short answer

The Simple compiler requires significant modifications to handle new operators, syntax structures, and data types, requiring changes both in compilation logic and machine instruction sets.

Step-by-step solution

Understanding the Task

We need to modify a Simple compiler to handle a variety of new features, including new operators, case-insensitive variables, input/output for multiple variables, error handling, arrays, subroutines, loop structures, string handling, and floating-point number processing. Some of these modifications will require changes to the Simpletron Simulator, while others will not.

Step A: Modulus Operator

Add the modulus operator to the compiler by extending the parsing process to recognize `%` in let statements. Modify the Simpletron Simulator to support the new MXL (Machine Language) instruction for modulus, involving a command that divides one operand by another and outputs the remainder.

Step B: Exponentiation Operator

Extend the compiler to parse the `^` operator in let statements, modifying it to generate a Simpletron instruction for exponentiation. Similarly, add this new operation to the Simpletron Simulator, ensuring it calculates a power operation efficiently.

Step C: Case Insensitivity

Adjust the compiler to treat variable names in a case-insensitive manner. Convert all alphabetic characters in Simple statements to a uniform case (e.g., lowercase) during lexical analysis, ensuring 'A' is equivalent to 'a'.

Step D: Multiple Input Variables

Modify the compiler's input parsing capabilities to accept and process multiple variables in a single input statement, such as `input x, y`. Extend the lexical analysis and parsing logic to handle separations using commas.

Step E: Multiple Output Values

Update the compiler to allow parsing of a list of variables in a single print statement, like `print a, b, c`. Extend the syntax analysis to separate and process each variable.

Step F: Syntax Checking

Introduce syntax-checking logic to the compiler to catch and output informative error messages for incorrectly formatted Simple programs. Implement checks during the parsing phase to identify errors such as missing parentheses or operators.

Step G: Integer Arrays

Enhance the compiler to recognize and process arrays of integers. Implement syntax and memory allocation strategies to handle data in a contiguous block, indexed by variables.

Step H: Subroutines

Incorporate support for the `gosub` and `return` commands into the compiler, allowing definition and use of subroutines. Ensure the compiler generates machine code for call and return sequences, managing the control flow.

Step I: For Loop with Step Increment

Add parsing logic to the compiler for handling for-loops with a specified increment, such as `for x=2 to 18 step 2`. Generate looping code that maintains the sequence and incorporates steps properly.

Step J: For Loop with Default Increment

Adjust the compiler to process for-loops without a specified increment, defaulting to `1`, as in `for x=2 to 10`. Ensure correct machine code generation for these loops.

Step K: String Processing

Modify the compiler to process string inputs and outputs, involving changes in the Simpletron Simulator to handle string data. Implement encoding of strings using ASCII and design new machine instructions for outputting strings.

Step L: Floating-point Processing

Extend both the compiler and Simpletron Simulator to support floating-point arithmetic. Modify the data representation and instruction set to calculate with non-integer values, ensuring operations such as addition, subtraction, multiplication, and division are correctly executed.

Key concepts

Syntax Checking

Syntax checking is all about ensuring that a program adheres to the rules or syntax of a programming language. Think of it as the grammar police for your code.
In the context of a compiler, syntax checking helps detect errors in program statements before they are executed. This is critical in avoiding runtime errors.
When a compiler performs syntax checking, it looks for things like:

• Missing or mismatched parentheses
• Incorrect use of operators
• Variables that are not declared

Introducing syntax checking into a compiler involves examining the code during the parsing stage. If errors are found, the compiler generates error messages, pointing out where the syntax is incorrect. This serves as a guide for developers to fix issues before running their program.
Implementing good syntax checking is important for enhancing the robustness of a compiler and for providing helpful feedback to programmers.

Subroutines

Subroutines are like mini-programs within a larger program. They allow a block of code to be reused multiple times without having to rewrite it.
In compiler design, subroutines are generally implemented using commands like `gosub` and `return`. The `gosub` command temporarily transfers control to a subroutine until a `return` command is encountered, when control is transferred back.
Subroutines help in:

• Code modularity
• Ease of maintenance
• Reusability
• Reduced redundancy

When a compiler encounters a `gosub` command, it generates machine code to store the current position in the program. When the subroutine is complete, the `return` command retrieves this position and resumes execution.
Understanding subroutines is crucial for efficient programming as it reinforces the concepts of code reuse and efficient control flow.

Floating-point Processing

Floating-point processing allows computers to handle real numbers, making calculations with fractions or very large numbers possible.
Traditional computers primarily work with integers, but many applications require more precision, which is where floating-point processing comes in.
To implement floating-point, a compiler must:

• Support floating-point data types
• Introduce specific machine instructions for floating-point arithmetic
• Handle the storage of non-integer numbers efficiently

Floating-point arithmetic includes operations like addition, subtraction, multiplication, and division on real numbers. The challenge lies in accurately representing these numbers within the constraints of a computer's binary system.
By introducing floating-point processing, a compiler can vastly improve the mathematical capabilities of its programs, especially in scientific calculations and graphics processing.

String Processing

String processing is about managing sequences of characters in a program. This involves handling text data inputs and outputs, which is essential for many applications.
In compiler design, string processing lets the language interpret, store, and manipulate character data. The Simpletron Simulator, for instance, can be modified to handle strings by treating a string as a sequence of ASCII values.
This involves:

• Allocating contiguous memory for strings
• Interpreting ASCII character codes
• Implementing string-handling instructions

String processing capability allows the compiler to manage tasks like formatting text, comparing strings, or extracting substrings without having to manually decode them.
With string processing support, the productivity and functionality of a programming language are significantly enhanced, making it easier to manage text data efficiently.