Assembly Language for Beginners

3.23. MORE ABOUT STRUCTURES

Figure 3.4:High score table

Now we can see that the file has changed after we added our name isBLSCORE.DAT.

% xxd -g 1 BLSCORE.DAT

00000000: 0a 00 58 65 6e 69 61 2e 2e 2e 2e 2e 00 df 01 00 ..Xenia.........
00000010: 00 30 33 2d 32 37 2d 32 30 31 38 00 50 61 75 6c .03-27-2018.Paul
00000020: 2e 2e 2e 2e 2e 2e 00 61 01 00 00 30 33 2d 32 37 .......a...03-27
00000030: 2d 32 30 31 38 00 4a 6f 68 6e 2e 2e 2e 2e 2e 2e -2018.John......
00000040: 00 46 01 00 00 30 33 2d 32 37 2d 32 30 31 38 00 .F...03-27-2018.
00000050: 4a 61 6d 65 73 2e 2e 2e 2e 2e 00 44 01 00 00 30 James......D...0
00000060: 33 2d 32 37 2d 32 30 31 38 00 43 68 61 72 6c 69 3-27-2018.Charli
00000070: 65 2e 2e 2e 00 ea 00 00 00 30 33 2d 32 37 2d 32 e........03-27-2
00000080: 30 31 38 00 4d 69 6b 65 2e 2e 2e 2e 2e 2e 00 b5 018.Mike........
00000090: 00 00 00 30 33 2d 32 37 2d 32 30 31 38 00 50 68 ...03-27-2018.Ph
000000a0: 69 6c 2e 2e 2e 2e 2e 2e 00 ac 00 00 00 30 33 2d il...........03-
000000b0: 32 37 2d 32 30 31 38 00 4d 61 72 79 2e 2e 2e 2e 27-2018.Mary....
000000c0: 2e 2e 00 7b 00 00 00 30 33 2d 32 37 2d 32 30 31 ...{...03-27-201
000000d0: 38 00 54 6f 6d 2e 2e 2e 2e 2e 2e 2e 00 77 00 00 8.Tom........w..
000000e0: 00 30 33 2d 32 37 2d 32 30 31 38 00 42 6f 62 2e .03-27-2018.Bob.
000000f0: 2e 2e 2e 2e 2e 2e 00 77 00 00 00 30 33 2d 32 37 .......w...03-27
00000100: 2d 32 30 31 38 00 -2018.

All entries are clearly visible. The very first byte is probably number of entries. Second is zero and, in fact,
number of entries can be 16-bit value spanning over first two bytes.

Next, after “Xenia” name we see 0xDF and 0x01 bytes. Xenia has score of 479, and this is exactly 0x1DF
in hexadecimal radix. So a high score value is probably 16-bit integer, or maybe 32-bit integer: there are
two more zero bytes after.

Now let’s think about the fact that both array elements and structure elements are always placed in
memory in adjacently to each other. That enables us to write the whole array/structure to the file using
simplewrite()orfwrite()function, and then restore it usingread()orfread(), as simple as that. This is
what is calledserializationnowadays.

Read

Now let’s write C program to read highscore file:

#include <assert.h>
#include <stdio.h>
#include <stdint.h>
#include <string.h>

struct entry
{