Code Explanation

The provided code is written in assembly language for a hypothetical architecture that manipulates memory contents to manage input and display functionality. It comprises two primary tasks: clearing the display and painting digits based on input. Below is a detailed breakdown of the code's components.

Main Program

JnS SubClearDisplay
JnS SubPaintDigit
Halt

Explanation

• JnS SubClearDisplay: This instruction calls the SubClearDisplay subroutine to clear the screen.
• JnS SubPaintDigit: Following the display clearance, this instruction invokes the SubPaintDigit subroutine to handle digit painting based on user input.
• Halt: Terminates the program execution.

---

SubClearDisplay

SubClearDisplay, HEX 0
loop, Load White
      StoreI ColCounter
      Load ColCounter
      Add one
      Store ColCounter
      Load ColCounter
      Subt LastCol
      Skipcond 800
      Jump loop
      JumpI SubClearDisplay

Key Components

• SubClearDisplay: Label defining the start of the subroutine.
• loop: A label indicating a continuous loop to fill the screen with a white color.
• Load White: Loads the value representing the white color from memory.
• StoreI ColCounter: Stores the white value at the memory location referenced by ColCounter.
• ColCounter: Used to iterate through columns; initialized to point to the first column.
• Add one: Increments ColCounter to point to the next column.
• Subt LastCol: Subtracts the last column index from ColCounter.
• Skipcond 800: Skips the next instruction if the result of Subt is zero, indicating the last column is reached.
• Jump loop: Returns to the beginning of the filling loop.
• JumpI SubClearDisplay: Returns from the subroutine to the instruction following the JnS call.

Summary

This subroutine iteratively fills every column of the display memory with the color white, continuing until all columns are filled.

---

SubPaintDigit

PaintDigitCharacter, HEX 0
PaintDigitDisplay, HEX 0
SubPaintDigit, HEX 0
                Input
                Store Number
                Skipcond 400
                JnS Selector

paintLoop,     LoadI FontAddr
                Store PaintDigitCharacter
                Load FontAddr
                Add One
                Store FontAddr

                LoadI Disp
                Store PaintDigitDisplay
                Load Disp
                Add One
                Store Disp

                Load PaintDigitCharacter //??
                StoreI PaintDigitDisplay //??

                LoadI Disp //?
                Subt One
                Skipcond 400
                Jump paintLoop
                JumpI SubPaintDigit

Key Components

• SubPaintDigit: Label indicating the start of the digit painting subroutine.
• Input / Store Number: Receives user input and stores it in the Number variable.
• Skipcond 400 / JnS Selector: If the input number is non-zero, it calls the Selector subroutine to handle specific digit logic.
• paintLoop: A loop to process the character representation for the display.
• LoadI FontAddr: Loads the current font address for the character to be displayed.
• Store / Add instructions: Update the font address and display address to paint the digit.
• StoreI PaintDigitDisplay: Stores the character representation at the display address.
• Jump paintLoop: Continues processing until all pixel data for the digit is displayed.

Summary

SubPaintDigit processes the digit input by loading the corresponding pixel data from a font table and sequentially storing it in the display memory, effectively rendering the digit.

---

Selector Subroutine

Selector,        HEX 0
Selectorloop,    Load FontAddr
                  Add 15
                  Store FontAddr
                  Load Number
                  Subt One
                  Store Number
                  Skipcond 400
                  Jump Selectorloop
                  JumpI Selector

Key Components

• Selectorloop: A loop that adjusts the FontAddr to the correct font representation based on the digit input.
• Add 15: A shift in the FontAddr that correlates to character dimensions, accommodating for each character being a certain width.
• Skipcond 400 / Jump Selectorloop: Repeats until all relevant font addresses are determined for the input number before returning.

Summary

This subroutine modifies the font pointer based on the input digit and is crucial for correctly rendering the digits on the screen.

---

Data Definitions

Memory Addresses and HEX Values

The latter section of the code defines data storage:

• FontAddr / Display: Base addresses in memory where font data and display memory begins.
• Film instructions and array representations: Each character (0-9) is represented with hex values to determine pixel representations.

Summary

The character rendering involves structured data, where 3 pixels wide and 5 pixels high representations are defined using hexadecimal values.

---

Conclusion

This assembly code implements a basic digit display system for a screen. It revolves around two main functionalities—clearing the screen and painting the digits based on user input. Understanding this code provides insight into low-level memory manipulation and routine-based programming in assembly language.

For individuals interested in enhancing their skills in assembly language or data manipulation techniques, consider exploring resources available at Enterprise DNA's Platform, which can provide further learning opportunities.