I know it might be and obvious question, but I've decided to learn a little bit of low level programming. I began with c and the gdb.
`(gdb) x/10xb $rip 0x4005a5 <main+4>: 0xb9 0x04 0x00 0x00 0x00 0xba 0x03 0x00 0x4005ad <main+12>: 0x00 0x00 (gdb) x/10xh $rip 0x4005a5 <main+4>: 0x04b9 0x0000 0xba00 0x0003 0x0000 0x02be 0x0000 0xbf00 0x4005b5 <main+20>: 0x0001 0x0000 (gdb) x/10xw $rip 0x4005a5 <main+4>: 0x000004b9 0x0003ba00 0x02be0000 0xbf000000 0x4005b5 <main+20>: 0x00000001 0xffff9fe8 0x0000b8ff 0xc35d0000 `
Question: Why when I use unit size b the next address is 0x4005ad but when I use h or w the next address is 0x4005b5?
`(gdb) x/4xw $rip + 0 0x4005a5 <main+4>: 0x000004b9 0x0003ba00 0x02be0000 0xbf000000 (gdb) x/4xw $rip + 1 0x4005a6 <main+5>: 0x00000004 0x000003ba 0x0002be00 0x01bf0000 (gdb) x/4xw $rip + 2 0x4005a7 <main+6>: 0xBA000000 0x00000003 0x000002be 0x0001bf00 (gdb) x/4xw $rip + 3 0x4005a8 <main+7>: 0x03BA0000 0xbe000000 0x00000002 0x000001bf (gdb) x/4xw $rip + 4 0x4005a9 <main+8>: 0x0003BA00 0x02be0000 0xbf000000 0x00000001 (gdb) x/4xw $rip + 5 0x4005aa <main+9>: 0x000003BA 0x0002be00 0x01bf0000 0xe8000000 (gdb) x/4xw $rip + 6 0x4005ab <main+10>: 0x00000003 0x000002be 0x0001bf00 0x9fe80000 (gdb) x/4xw $rip + 7 0x4005ac <main+11>: 0xBE000000 0x00000002 0x000001bf 0xff9fe800 (gdb) x/4xw $rip + 8 0x4005ad <main+12>: 0x02BE0000 0xbf000000 0x00000001 0xffff9fe8`
Question: Why the same value is repeated(Capital letters) for example: in the first column but move to the right, like from $rip + 2 to $rip + 5 where BA is first at the beginning, then at the middle and finally at the end?
When you ask any low-level debugger to display values from memory startingat a given address it will get some number of bytes from successive locations and display them. (Each address refers to a particular byte in memory)
In your first problem you're asking it to display ten bytes and it displays each byte as a two-digit hexadecimal value, eight bytes per line, and the address goes from 0x4005a5 to (0x4005a55 + 8) or 0x4005ad
Then you ask to display ten half words, eight half words per line, and because each half word is two bytes, the address goes from 0x4005a5 to (0x4005b5 + 16) or 0x4005b5
Your second problem is a little more complicated. Remember that when you ask it to display the contents of memory starting at a location it just fetches your four words STARTING at that location. When you pick one higher address then you're mostly getting the same memory values, just shifted by one.
So why do the values in the words seem to be shifting around in the wrong direction? That has to do with the fact that you're asking for words and x86 CPUs fetch words in a somewhat unintuitive order, from least significant byte to most.
This should help: https://en.wikipedia.org/wiki/Endianness
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