Поменять местами блоки данных в файле

var
f1,f2:file;
M1,M2,N,b:byte;
buf:array of byte;
begin
assign(f1,'C:\Pascal\filename1.txt');
reset(f1,1);
assign(f2,'C:\Pascal\filename2.txt');
reset(f2,1);
B:=0;
writeln('Enter 1st block');
read(M1);
writeln('Enter 2nd block');
read(M2);
writeln('Enter size of a block');
read(N);
Setlength(buf,N);
while (b<M1) and not Eof(f1) do
begin
blockread(f1,buf[0],1);
blockwrite(f2,buf[0],1);
b:=b+1;
end;
b:=0;
seek(f1,M2);
while(b<M2+N) and not Eof(f1) do
begin
blockread(f1,buf[0],1);
blockwrite(f2,buf[0],1);
b:=b+1;
end;
b:=0;
seek(f1,M1+N);
while(b<M2) and not Eof(f1) do
begin
blockread(f1,buf[0],1);
blockwrite(f2,buf[0],1);
b:=b+1
end;
b:=0;
seek(f1,M1);
while (b<M1+N) and not EoF(f1) do
begin
blockread(f1,buf[0],1);
blockwrite(f2,buf[0],1);
b:=b+1;
end;
b:=0;
seek(f1,M2+N);
while not EoF(f1) do
begin
blockread(f1,buf[0],1);
blockwrite(f2,buf[0],1);
b:=b+1;
end;
closefile(f1);
closefile(f2);
end.