Computer Programming

As described in the previous section, modern computers have a limited instruction set that largely concerns moving binary data between memory and CPU registers, and performing mathematical operations on data in those registers. Programs are expressed in ‘assembly code’ or the binary equivalent ‘machine code’ and are essentially sequences of those commands. For example, a program to print the line “Hello World” looks like:

global    _start

          section   .text
_start:   mov       rax, 1                  ; system call for write
          mov       rdi, 1                  ; file handle 1 is stdout
          mov       rsi, message            ; address of string to output
          mov       rdx, 13                 ; number of bytes
          syscall                           ; invoke operating system to do the write
          mov       rax, 60                 ; system call for exit
          xor       rdi, rdi                ; exit code 0
          syscall                           ; invoke operating system to exit

          section   .data
message:  db        "Hello, World", 10      ; note the newline at the end

Writing programs in assembly code is a considerable challenge. Thankfully, visionary programmers like Grace Hopper, Betty Holberton, and Adele Mildred Koss realized they could use programs to write portions of other programs. Grace Hopper took this to the next logical step, writing a program that could translate a more human-friendly language into machine code, known as FLOW-MATIC.

In her later contributions to COBOL, Grace Hopper successfully advocated for expressing program instructions in English. This concept of compiling a human-readable programming language (a higher-order programming language) into machine code is the basis of all modern programming languages: C/C++, Java, Python, JavaScript, C#, etc.

Thus, the same program as above expressed in Python becomes:

print("Hello, World")

Much easier to understand!

Programming languages have continued to evolve, introducing new syntax and organizational approaches intended to make it easier to write programs, reduce errors, improve security, and encourage good programming practices.

One advance in programming languages that is of especial interest to teachers are block-based programming languages like Scratch and Blockly. Instead of typing the program source code, in these languages you assemble the code by connecting graphical blocks. This approach avoids the possibility of an entire family of syntax errors, as the blocks will only snap together in ways that the compiler can interpret. As such, it makes learning to program more approachable for novices.

The same program as above in Scratch is:

Block-based programming is a great starting place for learning programming and computer science. But writing code by dragging and dropping blocks is also slower than typing, and eventually most programmers will transition to learning a text-based language.