Snake

Written in MIPS Assembly Language

Run Instructions

open MARS 4.4
open Keyboard and LED Display Simuator from TOOLS
connect LED window to MIPS
open and assemble snake.asm
run
begin game by tapping d or the right arrow key

Global Variables

$t5 = FRONT // begining of memory address for snake body
$t6 = END // end of memory address for snake body
$t7 = TAIL // address of tail
$t8 = HEAD // address of head
$t9 = total score
$s3 = time // time played in milliseconds
$s4 = score // temporary score
$s6 = button map // value of input from button presses
$s7 = length // logical size of snake

LED Values

00: off
01: red
10: yellow
11: green

Button Values

0xE0: UP
0XE1: DOWN
0XE2: LEFT
0XE3: RIGHT
0X42: B BUTTON

Snake Data Structure

ADT: Queue
IMPLEMENTATION: Circular Buffer

PART I: Initialization

_loadWalls

uses a series of loops that increment the x and y coordinates to build the walls for the board

void _loadWalls(int x, int y)
//	arguments: $a0 is x, $a1 is y
//	returns: none

_initSnake

initiates the snake data structure and sets the yellow LEDs on the board for the snake body. Also, finds initial length, tail address, and head address.

void _initSnake()
//	arguments: none
//	returns: none

_initFrogs

populates the board with 32 green frogs placed random location, using the _getRand method

void _initFrogs()
//	arguments: none
//	returns: none

PART 2: Main Loop

tick

sleep syscall for 200 milliseconds. Serves as our clock tick between moves. This makes the game playable.

void tick()
// 	arguments: none
// 	returns: none

_getButtonPress

Retrieves new button value from memory address if a button has been pressed. Otherwise the button value remains the same from the last tick. This function also deals with when the user tries to move in a direction opposite the one they're moving, ie. trying to move up when they're moving down. To deal with with, we add the last button value with the current value and mask off the last bit. If the bit is a 1 or 5, then the new button press is in the opposite direction and we don't assign the new button value by jumping to the end.

byte _getButtonPress()
//	arguments: none
//	returns: $t4 is button value

_peek

fetches and returns the coordinate from the head of the snake (end of queue)

half _peek()
//	arguments: none
//	returns: $v0 is x, $v1 is y

_remove

removes and returns the coordinate from the tail of the snake (front of the queue)

half _remove()
//	arguments: none
//	returns: $v0 is x, $v1 is y

_insert

inserts coordinate at the head of the snake (end of queue). Appends coordinate to head of snake and lights up LED.

void _insert(int x, int y)
//	arguments: $a0 is x, $a1 is y
//	returns: none

_getRand

randomly generates an integer between 0 and 63

byte _getRand()
//	arguments: none
//	returns: $v0 is random integer

_setLED

sets the LED at coordinate (x,y) to the value of $a2

void _setLED(int x, int y)
//	arguments: $a0 is x, $a1 is y
//	returns: none

_getLED

returns the LED value at coordinate (x,y)

byte _getLED(int x, int y)
//	arguments: $a0 is x, $a1 is y
//	returns: $v0 is LED value

events

This is the action function that moves the snake. From within events, we call the _move functions to move in different directions based on the button value. This is called after we tick the clock and retrieve that status of the button in the main loop. We implement movement by inserting at the end of the queue and removing from the front of the queue.

void events(int value)
//	arguments: $t4 is button value (direction)
//	returns: none

_moveRight

checks the space to the right of the head. If the coordinate is a wall, it reacts accordingly. Else if the coordinate is a frog, then it is eaten by the snake. Else if the coordinate is the snake body, then the game is over and we jump to the exit loop. Else the snake moves to the spot.

byte _moveRight()
//	arguments: none
//	returns: $t4 is button value (direction)

_moveLeft

checks the space to the left of the head. If the coordinate is a wall, it reacts accordingly. Else if the coordinate is a frog, then it is eaten by the snake. Else if the coordinate is the snake body, then the game is over and we jump to the exit loop. Else the snake moves to the spot.

byte _moveLeft()
//	arguments: none
//	returns: $t4 is button value (direction)

_moveUp

checks the space above the head. If the coordinate is a wall, it reacts accordingly. Else if the coordinate is a frog, then it is eaten by the snake. Else if the coordinate is the snake body, then the game is over and we jump to the exit loop. Else the snake moves to the spot.

With moveUp, we also check to see if we're moving through the slot in the top wall. If so, we add 63 to our coordinates to send it through the hole at the bottom wall.

byte _moveUp()
//	arguments: none
//	returns: $t4 is button value (direction)

_moveDown

checks the space below the head. If the coordinate is a wall, it reacts accordingly. Else if the coordinate is a frog, then it is eaten by the snake. Else if the coordinate is the snake body, then the game is over and we jump to the exit loop. Else the snake moves to the spot.

With moveDown, we check to see if we're moving through the hole in the bottom wall. If so, we subtract 63 from the y coordinate to send it through the hole in the top wall.

byte _moveDown()
//	arguments: none
//	returns: $t4 is button value (direction)

PART 3: Exit Loop

The exit loop prints both the time spent in the game and the total points to the console. The program is then terminated.

michaelplazek / SNAKE