Code Explainer

Assembly Code for Digit Rendering on Graphical Display

This assembly-like code snippet features subroutines for input handling and graphical representation, enabling the rendering of digits on a display by manipulating memory and using pixel data structures.

Prompt

JnS SubPaintDigit
Halt

//TAsk 2
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
                    
Selector,          HEX 0
Selectorloop,	   Load FontAddr
                   Add 15
                   Store FontAddr
                   Load Number
                   Subt One
                   Store Number
                   Skipcond 400
                   Jump Selectorloop
                   JumpI Selector
                 

Disp,              Adr Display       
Display,           HEX 0F11
                   HEX 0F12
                   HEX 0F13
                   HEX 0F21
                   HEX 0F22
                   HEX 0F23
                   HEX 0F31
                   HEX 0F32
                   HEX 0F33
                   HEX 0F41
                   HEX 0F42
                   HEX 0F43
                   HEX 0F51
                   HEX 0F52
                   HEX 0F53 
One,               HEX 1
Number,            HEX 0
column, 		  HEX 0F53

///////////////////////////////////////////////
//// Data
////
//// You don't need to change or add anything below
FontAddr, Adr Font
/ Each character is 3 pixels wide and 5 pixels high.
/ It is represented as 15 consecutive words of memory,
/ in "row-major order". That means that the first three
/ words represent the top-most row of pixels, the next
/ three words the second row, and so on.

Font, HEX FFFF       / Start of character 0
      HEX 0
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX 0
      HEX 0
      HEX FFFF
      HEX 0
      HEX 0
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX FFFF       / Start of character 1
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX 0
      HEX 0
      HEX FFFF
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX FFFF
      HEX 0
      HEX 0          / Start of character 2
      HEX 0
      HEX FFFF
      HEX FFFF
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX FFFF
      HEX 0
      HEX 0
      HEX 0
      HEX 0          / Start of character 3
      HEX 0
      HEX FFFF
      HEX FFFF
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX FFFF
      HEX FFFF
      HEX 0
      HEX 0
      HEX 0
      HEX FFFF
      HEX 0          / Start of character 4
      HEX FFFF
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX 0
      HEX 0
      HEX 0
      HEX 0
      HEX FFFF
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX FFFF
      HEX 0
      HEX 0          / Start of character 5
      HEX 0
      HEX 0
      HEX 0
      HEX FFFF
      HEX FFFF
      HEX 0
      HEX 0
      HEX FFFF
      HEX FFFF
      HEX FFFF
      HEX 0
      HEX 0
      HEX 0
      HEX FFFF
      HEX FFFF       / Start of character 6
      HEX 0
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX FFFF
      HEX 0
      HEX 0
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX 0          / Start of character 7
      HEX 0
      HEX 0
      HEX FFFF
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX FFFF
      HEX FFFF       / Start of character 8
      HEX 0
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX FFFF       / Start of character 9
      HEX 0
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX 0
      HEX 0
      HEX FFFF
      HEX FFFF
      HEX 0
      HEX FFFF
      HEX 0
      HEX FFFF

Answer

Code Overview

The code snippet provided is written in assembly-like language, featuring subroutines for rendering a digit on a graphical display. The general aim of this code is to take a number as input, determine the corresponding graphical representation, and display it on a defined area of the screen.

Main Sections of Code

Halt Instruction
Subroutine Definitions
- PaintDigit
- Selector
Data Definitions
Graphics Representation Data

Detailed Breakdown

1. Halt Instruction

Halt

This instruction stops the execution of the program. It is often used to signify the end of a main routine or to prevent further actions from occurring.

2. Subroutine Definitions

PaintDigit Subroutine

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

Purpose: This section declares variables for storing the character to be painted (PaintDigitCharacter), the display address (PaintDigitDisplay), and the subroutine address (SubPaintDigit).
Input Handling: The code section hints that it expects an Input, which will be stored in Number.
Conditional Jump: Skipcond 400 checks if the Number is zero (the negative condition). If not, it jumps to Selector.

Paint Loop

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

Loading Font Data:
- Uses LoadI to get the address of the font data and store it in PaintDigitCharacter.
- Datapoint for the font is incremented each loop iteration via Add One.
Updating Display:
- PaintDigitDisplay gets updated with the address at the current Disp, which is subsequently incremented.
Character Painting Loop:
- The PaintDigitCharacter is written to the memory location pointed at by PaintDigitDisplay.
- The loop continues until Disp is zero, indicated by Skipcond 400.

Selector Subroutine

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

Purpose: This routine is responsible for selecting the font address corresponding to the input number.
Font Address Calculation:
- Each character’s data consists of 15 memory addresses. It increments FontAddr accordingly.
Decrementing Number: The Number variable is decremented until it reaches zero, which signifies complete processing.

3. Data Definitions

Disp, Adr Display    
Display, HEX 0F11
...
FontAddr, Adr Font

Display Address: Disp is initialized to point to the starting addresses of the display area in memory.
Stored Hexadecimal Values: The sequence of HEX values under Display and Font outlines the pixel representations of characters.

4. Graphics Representation Data

Font Data:

Font, HEX FFFF ... HEX 0

Each character is designed in a matrix (3x5 for each character), and the data indicates on-off pixel states, where FFFF represents pixels being on.

Key Concepts

Subroutine (JnS)

Identification: The JnS (Jump and Save) instruction invokes a subroutine while preserving the return address.
Usage in Code: It is employed to navigate to the Selector for processing the number input before returning to PaintDigit.

Control Flow

Conditional Execution: Skipcond checks a condition before executing subsequent instructions, functioning as a decision point for the flow of the program.

Memory Handling

Indirect Addressing: LoadI and StoreI refer to loading and storing data through indirect addressing, allowing flexibility in accessing value locations in memory.

Representation of Graphical Data

Graphical representation for digits is accomplished via pixel data stored in memory, which is referenced in the main processing loop.

Conclusion

This assembly-like code provides a procedural approach to display a numerical digit on a graphical interface by leveraging subroutines for modular processing and using hexadecimals to manipulate character drawings in a pixel matrix format. Understanding these components and their interactions is crucial for effective application in graphical programming and assembly language.