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| Rev | Author | Line No. | Line |
|---|---|---|---|
| 385 | savelij | 1 | |
| 2 | .SNA, .snap or .snapshot |
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| 3 | (Mirage Microdrive format used by many emulators) |
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| 4 | |||
| 5 | This format is the most well-supported of all snapshot formats (though Z80 is |
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| 6 | close on its heels) but has a drawback: |
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| 7 | |||
| 8 | As the program counter is pushed onto the stack so that a RETN instruction can |
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| 9 | restart the program, 2 bytes of memory are overwritten. This will usually not |
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| 10 | matter; the game (or whatever) will have stack space that can be used for this. |
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| 11 | However, if this space is all in use when the snap is made, memory below the |
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| 12 | stack space will be corrupted. According to Rui Ribeiro, the effects of this can |
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| 13 | sometimes be avoided by replacing the corrupted bytes with zeros; e.g. take the |
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| 14 | PC from the, stack pointer, replace that word with 0000 and then increment SP. |
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| 15 | This worked with snapshots of Batman, Bounder and others which had been saved at |
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| 16 | critical points. Theoretically, this problem could cause a complete crash |
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| 17 | on a real Spectrum if the stack pointer happened to be at address 16384; |
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| 18 | the push would try and write to the ROM. How different emulators handle this |
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| 19 | is not something I know... |
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| 20 | |||
| 21 | When the registers have been loaded, a RETN command is required to start the |
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| 22 | program. IFF2 is short for interrupt flip-flop 2, and for all practical purposes |
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| 23 | is the interrupt-enabled flag. Set means enabled. |
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| 24 | |||
| 25 | Offset Size Description |
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| 26 | ------------------------------------------------------------------------ |
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| 27 | |||
| 28 | 1 8 word HL',DE',BC',AF' |
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| 29 | 9 10 word HL,DE,BC,IY,IX |
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| 30 | 19 1 byte Interrupt (bit 2 contains IFF2, 1=EI/0=DI) |
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| 31 | 20 1 byte R |
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| 32 | 21 4 words AF,SP |
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| 33 | 25 1 byte IntMode (0=IM0/1=IM1/2=IM2) |
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| 34 | 26 1 byte BorderColor (0..7, not used by Spectrum 1.7) |
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| 35 | 27 49152 bytes RAM dump 16384..65535 |
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| 36 | ------------------------------------------------------------------------ |
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| 37 | Total: 49179 bytes |
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| 38 | |||
| 39 | .SNA (128Kb version) (SP_EMU) |
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| 40 | |||
| 41 | This is simply the SNA format extended to include the extra memory banks of the |
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| 42 | 128K/+2 machines, and fixes the problem with the PC being pushed onto the stack |
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| 43 | - now it is located in an extra variable in the file (and is not pushed onto the |
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| 44 | stack at all). The first 49179 bytes of the snapshot are otherwise exactly as |
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| 45 | described above, so the full description is: |
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| 46 | |||
| 47 | Offset Size Description |
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| 48 | ------------------------------------------------------------------------ |
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| 49 | |||
| 50 | 27 16Kb bytes RAM bank 5 \ |
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| 51 | 16411 16Kb bytes RAM bank 2 } - as standard 48Kb SNA file |
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| 52 | 32795 16Kb bytes RAM bank n / (currently paged bank) |
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| 53 | 49179 2 word PC |
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| 54 | 49181 1 byte port 7FFD setting |
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| 55 | 49182 1 byte TR-DOS rom paged (1) or not (0) |
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| 56 | 49183 16Kb bytes remaining RAM banks in ascending order |
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| 57 | ... |
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| 58 | ------------------------------------------------------------------------ |
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| 59 | Total: 131103 or 147487 bytes |
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| 60 | |||
| 61 | The third RAM bank saved is always the one currently paged, even if this is |
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| 62 | page 5 or 2 - in this case, the bank is actually included twice. The remaining |
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| 63 | RAM banks are saved in ascending order - e.g. if RAM bank 4 is paged in, the |
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| 64 | snapshot is made up of banks 5, 2 and 4 to start with, and banks 0, 1, 3, 6 |
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| 65 | and 7 afterwards. If RAM bank 5 is paged in, the snapshot is made up of |
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| 66 | banks 5, 2 and 5 again, followed by banks 0, 1, 3, 4, 6 and 7. |