エラーメッセージの解釈・メモリの節約方法

##不明点
プログラミングについて質問です．
fortranを用いて数値計算を行ったところ，以下のメッセージが出てきました．

Program received signal SIGBUS: Access to an undefined portion of a memory object.

Backtrace for this error:
0 0x1046e4b17
1 0x1046e3bab
2 0x1b30544e3
3 0x1042ae27f
zsh: bus error ./a.out

このメッセージは「メモリが足りない」ということだと思いますが，この問題を解決するには
（1）RAMの容量の大きなパソコン（計算機）で計算する
（2）ソースコードの見直しによってメモリを節約する
の2つの方法がある，という認識であっていますか？
また，ソースコードの見直しの場合，どこを変えれば節約することができるでしょうか？
どなたか詳しい方いましたらご教示いただけますか．
よろしくお願いいたします．

##環境
M1 MacBookAir
8GB

##ソースコード

Fortran
1      implicit real*8 (a-h,o-z)
2      parameter(nx=21, ny=21, nz=21)
3      common /aray1/ temp(0:nx,0:ny,0:nz)
4      common /aray2/ qmin(0:nx,0:ny),qmax(0:nx,0:ny)
5      common /coor1/ tinf, rho, ep, sig, cp, cond, alpha,time,plp,pp
6      common /coor2/ imax, jmax, kmax, nstep, nout
7      common /coor3/ dd, dt, dx, dy, dz, phi, ss, cfl, eta, tmax, tm
8      common /coor4/ ff
9      common /coor5/ x1, y1, z1, rs
10 
11
12      call parm
13      call init
14
15      open(18, file='mydata.csv', form='formatted', status='unknown')
16      
17c main sequence
18      locl = 0
19      time = 0.0
20 1000 continue
21      locl = locl + 1
22      loop =loop + 1
23      time = time + dt
24 
25      call intg
26      call rbdy
27 
28c post sequence
29      m =mod(locl, nout)
30      if(m.eq.0) then
31         call clth
32         call wrtd
33         write(18,*) time,',',ff,',',
34     &    temp(int(imax/2),int(jmax/2),int(kmax/2))
35      endif
36      if(locl.lt.nstep) go to 1000
37      close(18)
38      
39
40c exit
41      stop
42      end
43
44      
45c----------------------------------------------------------------     
46      subroutine parm
47
48      implicit real*8 (a-h,o-z)
49      parameter(nx=21, ny=21, nz=21)
50      common /aray1/ temp(0:nx,0:ny,0:nz)
51      common /coor1/ tinf, rho, ep, sig, cp, cond, alpha,time,plp,pp
52      common /coor2/ imax, jmax, kmax, nstep, nout
53      common /coor3/ dd, dt, dx, dy, dz, phi, ss, cfl, eta, tmax, tm
54      common /coor5/ x1, y1, z1, rs
55
56      tmax=2.0
57      dd=0.00163
58      xmax=0.002
59      ymax=0.002
60      zmax=0.002
61      dx=xmax/nx
62      dy=ymax/ny
63      dz=zmax/nz
64      x1=xmax/2
65      y1=ymax/2
66      z1=zmax/2
67      tinf=273.0+1850.0
68      phi=0.0007
69      ss=0.25*3.1414*phi**2
70      cfl=0.5
71      eta=0.121
72      nout = 100
73      tm=1850.0+273.0
74      rs=dd/2
75
76      return
77      end
78      
79c----------------------------------------------------------------     
80      subroutine init
81 
82      implicit real*8 (a-h,o-z)
83      parameter(nx=21, ny=21, nz=21)
84      common /aray1/ temp(0:nx,0:ny,0:nz)
85      common /aray2/ qmin(0:nx,0:ny),qmax(0:nx,0:ny)
86      common /coor1/ tinf, rho, ep, sig, cp, cond, alpha,time,plp,pp
87      common /coor2/ imax, jmax, kmax, nstep, nout
88      common /coor3/ dd, dt, dx, dy, dz, phi, ss, cfl, eta, tmax, tm
89      common /coor5/ x1, y1, z1, rs
90      
91      rho=5800.0
92      ep=0.32
93      sig=5.67e-8
94      cp=436.0
95      cond=94.61
96      alpha=cond/(rho*cp)
97      dt=cfl*dx**2/2/alpha
98      write(6,*) dt
99      nstep=tmax/dt
100      
101      x=0.0
102      do i=0,imax
103         y=0.0
104         do j=0,jmax
105            z=0.0
106            cout=0
107            do k=0,kmax
108               if (z>=z1-dsqrt(rs**2-(y-y1)**2-(x-x1)**2) 
109     &          .and. z<=z1+dsqrt(rs**2-(y-y1)**2-(x-x1)**2)) then
110                       qmax(i,j)=k
111                       cout = cout +1
112               end if
113
114               if (z>=z1-dsqrt(rs**2-(y-y1)**2-(x-x1)**2) 
115     &          .and. z<=z1+dsqrt(rs**2-(y-y1)**2-(x-x1)**2)) then
116                  temp(i,j,k)=tinf
117               end if
118               z=z+dz
119            end do
120            qmin(i,j)=qmax(i,j)-cout+1
121            y=y+dy
122         end do
123         x=x+dx
124      end do
125
126      return
127      end
128
129c----------------------------------------------------------------     
130      subroutine intg
131
132      implicit real*8 (a-h,o-z)
133      parameter(nx=21, ny=21, nz=21)
134      common /aray1/ temp(0:nx,0:ny,0:nz)
135      common /coor1/ tinf, rho, ep, sig, cp, cond, alpha,time,plp,pp
136      common /coor2/ imax, jmax, kmax, nstep, nout
137      common /coor3/ dd, dt, dx, dy, dz, phi, ss, cfl, eta, tmax, tm
138      common /coor5/ x1, y1, z1, rs
139
140      if (time<0.3) then
141        plp = 0.0
142      else if (time<1.7) then
143        plp = 100.0
144      else 
145        plp = 0.0
146      end if
147      pp = plp / ss
148
149c     Euler explicit method
150      x=dx
151      do i=1,imax-1
152         y=dy
153         do j=1, jmax-1
154            z=dz
155            do k=1, kmax-1
156               cp = 436.0-0.0813*(temp(i,j,k)-tm)
157               cond = 94.61 + 4.41e-2*(temp(i,j,k)-tm)
158               if (z>=z1-dsqrt(rs**2-(y-y1)**2-(x-x1)**2) 
159     &          .and. z<=z1+dsqrt(rs**2-(y-y1)**2-(x-x1)**2)) then
160                  call rbdy
161                  temp(i,j,k) = temp(i,j,k) 
162     &              + dt*cond/rho/cp*(temp(i+1,j,k)
163     &              -2.0*temp(i,j,k)+temp(i-1,j,k))/(dx**2)
164     &              + dt*cond/rho/cp*(temp(i,j+1,k)
165     &              -2.0*temp(i,j,k)+temp(i,j-1,k))/(dx**2)
166     &              + dt*cond/rho/cp*(temp(i,j,k+1)
167     &              -2.0*temp(i,j,k)+temp(i,i,k-1))/(dx**2)
168               end if
169               z=z+dz
170            end do
171            y=y+dy
172         end do
173         x=x+dx
174      end do
175
176      do i=1,imax-1
177         do j=1,jmax-1
178            do k=1,kmax-1
179               write(6,*) temp(i,j,k)
180            end do
181         end do
182      end do
183
184      return
185      end
186
187c----------------------------------------------------------------     
188      subroutine rbdy
189
190      implicit real*8 (a-h,o-z)
191      parameter(nx=21, ny=21, nz=21)
192      common /aray1/ temp(0:nx,0:ny,0:nz)
193      common /aray2/ qmin(0:nx,0:ny),qmax(0:nx,0:ny)
194      common /coor1/ tinf, rho, ep, sig, cp, cond, alpha,time,plp,pp
195      common /coor2/ imax, jmax, kmax, nstep, nout
196      common /coor3/ dd, dt, dx, dy, dz, phi, ss, cfl, eta, tmax, tm
197      if (i.eq.qmin(j,k) .or. i.eq.qmax(j,k) .or. j.eq.qmin(i,k).or. 
198     &   j.eq.qmax(i,k) .or. k.eq.qmin(i,j) .or. k.eq.qmax(i,j)) then
199            if (i.eq.qmin(i,j)) then
200               temp(i-1,j,k)=temp(i,j,k)
201     &                    -dx*ep*sig*temp(i,j,k)**4/cond
202            end if
203            if (i.eq.qmax(i,j)) then
204               temp(i+1,j,k)=temp(i,j,k)
205     &                    -dx*ep*sig*temp(i,j,k)**4/cond
206            end if
207            if (j.eq.qmin(i,j)) then
208               temp(i,j-1,k)=temp(i,j,k)
209     &                    -dx*ep*sig*temp(i,j,k)**4/cond
210            end if
211            if (j.eq.qmax(i,j)) then
212               temp(i,j+1,k)=temp(i,j,k)
213     &                    -dx*ep*sig*temp(i,j,k)**4/cond
214            end if
215            if (k.eq.qmin(i,j)) then
216               temp(i,j,k-1)=temp(i,j,k)
217     &                    -dx*ep*sig*temp(i,j,k)**4/cond
218            end if
219            if (k.eq.qmax(i,j)) then
220               temp(i,j,k+1)=temp(i,j,k)
221     &        -dx*ep*sig*temp(i,j,k)**4/cond+eta*pp*dz/cond
222            end if
223      end if
224      return
225      end
226
227c----------------------------------------------------------------     
228      subroutine clth
229
230      implicit real*8 (a-h,o-z)
231      parameter(nx=21, ny=21, nz=21)
232      common /aray1/ temp(0:nx,0:ny,0:nz)
233      common /aray2/ qmin(0:nx,0:ny),qmax(0:nx,0:ny)
234      common /aray3/ ph(0:nx,0:ny),pl(0:nx,0:ny) 
235      common /coor1/ tinf, rho, ep, sig, cp, cond, alpha,time,plp,pp
236      common /coor2/ imax, jmax, kmax, nstep, nout
237      common /coor3/ dd, dt, dx, dy, dz, phi, ss, cfl, eta, tmax, tm
238      common /coor4/ ff
239      common /coor5/ x1, y1, z1, rs
240      
241      x=0.0
242      do i=0,imax
243         y=0.0
244         do j=0,jmax
245            z=0.0
246            pl(i,j)=0.0
247            ph(i,j)=0.0
248            do k=0,kmax
249               if (z>=z1-dsqrt(rs**2-(y-y1)**2-(x-x1)**2) 
250     &          .and. z<=z1+dsqrt(rs**2-(y-y1)**2-(x-x1)**2)) then
251                  if(k.eq.qmin(i,j) .or. k.eq.qmax(i,j)) then
252                     if(z>z1) then
253                        ph(i,j)=vapor(temp(i,j,k))
254                     else if(z<z1) then
255                        pl(i,j)=vapor(temp(i,j,k))
256                     end if
257                  end if
258               end if
259               z=z+dz
260            end do
261            y=y+dy
262         end do
263         x=x+dx
264      end do
265      
266      ff=0.0
267      do i=0,imax
268         y=0.0
269         do j=0,jmax
270            pres = ph(i,j)-pl(i,j)
271            ff=ff+pres*dx*dy
272         end do
273      end do
274
275      return
276      end
277
278c----------------------------------------------------------------     
279      subroutine wrtd
280
281      implicit real*8 (a-h,o-z)
282      parameter(nx=21, ny=21, nz=21)
283      common /coor1/ tinf, rho, ep, sig, cp, cond, alpha,time,plp,pp
284      common /coor2/ imax, jmax, kmax, nstep, nout
285      common /coor3/ dd, dt, dx, dy, dz, phi, ss, cfl, eta, tmax, tm
286      common /coor4/ ff
287      
288      return
289      end
290c----------------------------------------------------------------     
291      real(8) function vapor(y)
292         implicit none
293         real(8) :: y
294         vapor=10.0**(5.006+6.806-30295.0/y)
295         return
296      end function vapor