Alice Averlong🏳️⚧️why do I keep hacking 16bit DOS games? do I hate myself?
32bit programs are SO MUCH EASIER to RE, because when you see an address, you know what it means. 0x12345678 always means 0x12345678!
16bit games are full of MOV AX, 1234h and it's like, WHAT'S DS AT THIS POINT? WHICH 1234?
there's 65536 possible memory addresses it could be!
not to mention that there's more than one way to address a given part of memory.
in 32bit and 64bit code, if you see 0x12345678, you know that some code that writes to 0x12335662 doesn't change it.
not so in 16bit games. you have plenty of ways to refer to the same address.
This is why 16bit x86 is SO much more annoying than 8-bit computers.
with 8-bit computers, you have 16-bit addresses, because 256 bytes is rarely enough memory. So they work by having some addresses which are longer. simple, right? so instead of an 8bit number, you have a 16bit number.
with 8-bit computers, you have 16-bit addresses, because 256 bytes is rarely enough memory. So they work by having some addresses which are longer. simple, right? so instead of an 8bit number, you have a 16bit number.
16bit x86 does this as well. 16bits of ram is only 64kb, and that's just not enough. So you expand it to 24bits or 32bits, for "long addresses", right? same as you use in 8bit computers?
NOPE
segmented addressing, the solution they use, is not as simple as just adding some more bits. a 16bit segment and a 16bit offset.
so that's just a weird way of explaining a 32bit number, right?
NOPE
no, you combine 16bits and 16bits and get... 20 bits.
it's a 20bit address.
so what, they ignore all but the bottom 4 bits of the segment?
NO THAT WOULD MAKE SENSE
instead the full 16bit segment is used, but it's turned into a 20bit address by shifting it 4 bits over and adding in the offset.
So it's the TOP 4 bits that are important, not the bottom 4.
Okay that's fine, but wait, I said adding. Not "replacing".
Yes, all 16bits are used. So the address 0000:0000 is (linear) 0x0, and 0001:0000 is (linear) 0x10
which also means that 0001:0000 and 0000:0010 are both linear 0x10.
So you can get pointer aliasing even though both pointers HAVE DIFFERENT VALUES
Aaron Sawdey, Ph.D.@foone this is a little like indexed addressing modes and how an out of order core might not be able to resolve everything to identify load/store conflicts early in the pipeline.
Loving your rant because I always also thought that x86 segmented addressing was so stupid. The 386 as the first one that was nicely usable for *nix type OSes.