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                        .SNA, .snap or .snapshot 
                (Mirage Microdrive format used by many emulators)

This format is the most well-supported of all snapshot formats (though Z80 is 
close on its heels) but has a drawback:

As the program counter is pushed onto the stack so that a RETN instruction can 
restart the program, 2 bytes of memory are overwritten. This will usually not 
matter; the game (or whatever) will have stack space that can be used for this. 
However, if this space is all in use when the snap is made, memory below the 
stack space will be corrupted. According to Rui Ribeiro, the effects of this can 
sometimes be avoided by replacing the corrupted bytes with zeros; e.g. take the 
PC from the, stack pointer, replace that word with 0000 and then increment SP. 
This worked with snapshots of Batman, Bounder and others which had been saved at 
critical points. Theoretically, this problem could cause a complete crash 
on a real Spectrum if the stack pointer happened to be at address 16384; 
the push would try and write to the ROM. How different emulators handle this 
is not something I know...

When the registers have been loaded, a RETN command is required to start the 
program. IFF2 is short for interrupt flip-flop 2, and for all practical purposes 
is the interrupt-enabled flag. Set means enabled.

   Offset   Size   Description
   ------------------------------------------------------------------------
   0        1      byte   I
   1        8      word   HL',DE',BC',AF'
   9        10     word   HL,DE,BC,IY,IX
   19       1      byte   Interrupt (bit 2 contains IFF2, 1=EI/0=DI)
   20       1      byte   R
   21       4      words  AF,SP
   25       1      byte   IntMode (0=IM0/1=IM1/2=IM2)
   26       1      byte   BorderColor (0..7, not used by Spectrum 1.7)
   27       49152  bytes  RAM dump 16384..65535
   ------------------------------------------------------------------------
   Total: 49179 bytes

                        .SNA (128Kb version) (SP_EMU)

This is simply the SNA format extended to include the extra memory banks of the 
128K/+2 machines, and fixes the problem with the PC being pushed onto the stack
- now it is located in an extra variable in the file (and is not pushed onto the 
stack at all). The first 49179 bytes of the snapshot are otherwise exactly as 
described above, so the full description is:

   Offset   Size   Description
   ------------------------------------------------------------------------
   0        27     bytes  SNA header (see above)
   27       16Kb   bytes  RAM bank 5 \
   16411    16Kb   bytes  RAM bank 2  } - as standard 48Kb SNA file
   32795    16Kb   bytes  RAM bank n / (currently paged bank)
   49179    2      word   PC
   49181    1      byte   port 7FFD setting
   49182    1      byte   TR-DOS rom paged (1) or not (0)
   49183    16Kb   bytes  remaining RAM banks in ascending order
   ...
   ------------------------------------------------------------------------
   Total: 131103 or 147487 bytes

The third RAM bank saved is always the one currently paged, even if this is 
page 5 or 2 - in this case, the bank is actually included twice. The remaining 
RAM banks are saved in ascending order - e.g. if RAM bank 4 is paged in, the 
snapshot is made up of banks 5, 2 and 4 to start with, and banks 0, 1, 3, 6 
and 7 afterwards. If RAM bank 5 is paged in, the snapshot is made up of 
banks 5, 2 and 5 again, followed by banks 0, 1, 3, 4, 6 and 7.