Reverse Code Engineering RCE CD +sandman 2000/ReverseCodeEngineeringRceCdsandman2000.iso/RCE/Library/Manuels & Misc/Assembly/AOA.ZIP/CH10/MOON.ASM

Assembly Source File | 1996-02-08 | 8.1 KB | 362 lines

; Simple "Moon Lander" game.
;
; Randall Hyde
; 2/8/96
;
; This program is an example of a trivial little "moon lander"
; game that simulates a Lunar Module setting down on the Moon's
; surface.  At time T=0 the spacecraft's velocity is 1000 ft/sec
; downward, the craft has 1000 units of fuel, and the craft is
; 10,000 ft above the moon's surface.  The pilot (user) can
; specify how much fuel to burn at each second.
;
; Note that all calculations are approximate since everything is
; done with integer arithmetic.


; Some important constants

InitialVelocity    =    1000
InitialDistance    =    10000
InitialFuel    =    250
MaxFuelBurn    =    25

MoonsGravity    =    5        ;Approx 5 ft/sec/sec
AccPerUnitFuel    =    -5        ;-5 ft/sec/sec for each fuel unit.


        .xlist
        include     stdlib.a
        includelib    stdlib.lib
        .list

        .386            ;Comment out these two statements
        option    segment:use16    ; if you are not using an 80386.

dseg        segment    para public 'data'

; Current distance from the Moon's Surface:

CurDist        word    InitialDistance

; Current Velocity:

CurVel        word    InitialVelocity

; Total fuel left to burn:

FuelLeft    word    InitialFuel


; Amount of Fuel to use on current burn.

Fuel        word    ?

; Distance travelled in the last second.

Dist        word    ?

dseg        ends

cseg        segment    para public 'code'
        assume    cs:cseg, ds:dseg

; GETI-    Reads an integer variable from the user and returns its
;    its value in the AX register.  If the user entered garbage,
;    this code will make the user re-enter the value.

geti        textequ    <call _geti>
_geti        proc
        push    es
        push    di
        push    bx

; Read a string of characters from the user.
;
; Note that there are two (nested) loops here.  The outer loop
; (GetILp) repeats the getsm operation as long as the user
; keeps entering an invalid number.  The innermost loop (ChkDigits)
; checks the individual characters in the input string to make
; sure they are all decimal digits.

GetILp:        getsm

; Check to see if this string contains any non-digit characters:
;
; while (([bx] >= '0') and ([bx] <= '9')  bx := bx + 1;
;
; Note the sneaky way of turning the while loop into a
; repeat..until loop.

        mov    bx, di        ;Pointer to start of string.
        dec    bx
ChkDigits:    inc    bx
        mov    al, es:[bx]    ;Fetch next character.
        IsDigit            ;See if it's a decimal digit.
        je    ChkDigits    ;Repeat if it is.

        cmp    al, 0        ;At end of string?
        je    GotNumber

; Okay, we just ran into a non-digit character.  Complain and make
; the user reenter the value.

        free            ;Free space malloc'd by getsm.
        print
        byte    cr,lf
        byte    "Illegal unsigned integer value, "
        byte    "please reenter.",cr,lf
        byte    "(no spaces, non-digit chars, etc.):",0
        jmp    GetILp


; Okay, ES:DI is pointing at something resembling a number.  Convert
; it to an integer.

GotNumber:    atoi
        free            ;Free space malloc'd by getsm.

        pop    bx
        pop    di
        pop    es
        ret
_geti        endp





; InitGame-    Initializes global variables this game uses.

InitGame    proc
        mov    CurVel, InitialVelocity
        mov    CurDist, InitialDistance
        mov    FuelLeft, InitialFuel
        mov    Dist, 0
        ret
InitGame    endp


; DispStatus-    Displays important information for each
;        cycle of the game (a cycle is one second).

DispStatus    proc
        printf
        byte    cr,lf
        byte    "Distance from surface: %5d",cr,lf
        byte    "Current velocity:      %5d",cr,lf
        byte    "Fuel left:             %5d",cr,lf
        byte    lf
        byte    "Dist travelled in the last second: %d",cr,lf
        byte    lf,0
        dword    CurDist, CurVel, FuelLeft, Dist
        ret
DispStatus    endp


; GetFuel-    Reads an integer value representing the amount of fuel
;        to burn from the user and checks to see if this value
;        is reasonable.  A reasonable value must:
;
;        * Be an actual number (GETI handles this).
;        * Be greater than or equal to zero (no burning
;          negative amounts of fuel, GETI handles this).
;        * Be less than MaxFuelBurn (any more than this and
;          you have an explosion, not a burn).
;        * Be less than the fuel left in the Lunar Module.

GetFuel        proc
        push    ax

; Loop..endloop structure that reads an integer input and terminates
; if the input is reasonable.  It prints a message an repeats if
; the input is not reasonable.
;
; loop
;    get fuel;
;    if (fuel < MaxFuelBurn) then break;
;    print error message.
; endloop
;
; if (fuel > FuelLeft) then
;
;    fuel = fuelleft;
;    print appropriate message.
;
; endif

GetFuelLp:    print
        byte    "Enter amount of fuel to burn: ",0
        geti
        cmp    ax, MaxFuelBurn
        jbe    GoodFuel

        print
        byte    "The amount you've specified exceeds the "
        byte    "engine rating,", cr, lf
        byte    "please enter a smaller value",cr,lf,lf,0
        jmp    GetFuelLp

GoodFuel:    mov    Fuel, ax
        cmp    ax, FuelLeft
        jbe    HasEnough
        printf
        byte    "There are only %d units of fuel left.",cr,lf
        byte    "The Lunar module will burn this rather than %d"
        byte    cr,lf,0
        dword    FuelLeft, Fuel

        mov    ax, FuelLeft
        mov    Fuel, ax

HasEnough:    mov    ax, FuelLeft
        sub    ax, Fuel
        mov    FuelLeft, ax
        pop    ax
        ret
GetFuel        endp



; ComputeStatus-
;
;    This routine computes the new velocity and new distance based on the
;    current distance, current velocity, fuel burnt, and the moon's
;    gravity.  This routine is called for every "second" of flight time.
;
; note:
;
;    Distance Travelled = Acc*T*T/2 + Vel*T  (note: T=1, so it goes away).
;    Acc = MoonsGravity + Fuel * AccPerUnitFuel
;
;    New Velocity = Acc*T + Prev Velocity
;
;     This code should really average these values over the one second
;    time period, but the simulation is so crude anyway, there's no
;    need to really bother.

ComputeStatus    proc
        push    ax
        push    bx
        push    dx

; First, compute the acceleration value based on the fuel burnt
; during this second (Acc = Moon's Gravity + Fuel * AccPerUnitFuel).

        mov    ax, Fuel        ;Compute
        mov    dx, AccPerUnitFuel    ; Fuel*AccPerUnitFuel
        imul    dx

        add    ax, MoonsGravity    ;Add in Moon's gravity.
        mov    bx, ax            ;Save Acc value.

; Now compute the new velocity (V=AT+V)

        add    ax, CurVel        ;Compute new velocity
        mov    CurVel, ax

; Next, compute the distance travelled (D = 1/2 * A * T^2 + VT +D)

        sar    bx, 1            ;Acc/2
        add    ax, bx            ;Acc/2 + V (T=1!)
        mov    Dist, ax        ;Distance Travelled.
        neg    ax
        add    CurDist, ax        ;New distance.

        pop    dx
        pop    bx
        pop    ax
        ret
ComputeStatus    endp


; GetYorN-    Reads a yes or no answer from the user (Y, y, N, or n).
;        Returns the character read in the al register (Y or N,
;        converted to upper case if necessary).

GetYorN        proc
        getc
        ToUpper
        cmp    al, 'Y'
        je    GotIt
        cmp    al, 'N'
        jne    GetYorN
GotIt:        ret
GetYorN        endp




Main        proc
        mov    ax, dseg
        mov    ds, ax
        mov    es, ax
        meminit

MoonLoop:    print
        byte    cr,lf,lf
        byte    "Welcome to the moon lander game.",cr,lf,lf
        byte    "You must manuever your craft so that you touch"
        byte    "down at less than 10 ft/sec",cr,lf
        byte    "for a soft landing.",cr,lf,lf,0

        call    InitGame

; The following loop repeats while the distance to the surface is greater
; than zero.

WhileStillUp:    mov    ax, CurDist
        cmp    ax, 0
        jle    Landed

        call    DispStatus
        call    GetFuel
        call    ComputeStatus
        jmp    WhileStillUp

Landed:        cmp    CurVel, 10
        jle    SoftLanding

        printf
        byte    "Your current velocity is %d.",cr,lf
        byte    "That was just a little too fast.  However, as a "
        byte    "consolation prize,",cr,lf
        byte    "we will name the new crater you just created "
        byte    "after you.",cr,lf,0
        dword    CurVel

        jmp    TryAgain

SoftLanding:    printf
        byte    "Congrats!  You landed the Lunar Module safely at "
        byte    "%d ft/sec.",cr,lf
        byte    "You have %d units of fuel left.",cr,lf
        byte    "Good job!",cr,lf,0
        dword    CurVel, FuelLeft

TryAgain:    print
        byte    "Do you want to try again (Y/N)? ",0
        call    GetYorN
        cmp    al, 'Y'
        je    MoonLoop

        print
        byte    cr,lf
        byte    "Thanks for playing!  Come back to the moon again sometime"
        byte    cr,lf,lf,0


Quit:        ExitPgm            ;DOS macro to quit program.
Main        endp

cseg        ends

sseg        segment    para stack 'stack'
stk        byte    1024 dup ("stack   ")
sseg        ends

zzzzzzseg    segment    para public 'zzzzzz'
LastBytes    byte    16 dup (?)
zzzzzzseg    ends
        end    Main