Using ZX Spectrum Snapshots

A snapshot is a frozen ZX Spectrum: the Z80 registers, the RAM, and the state of the hardware around it (border, memory paging, the AY sound chip). Games and demos are often shared as snapshots, and a snapshot is also the easiest way to hand someone a machine stopped exactly where a bug shows up.

Klive reads the three common formats:

Viewing a snapshot

Open a .sna, .z80 or .szx file from the Explorer to see what it holds:

• Snapshot: the format, the machine it was saved on and what it loads as in Klive, whether Klive can load it (and if not, why), every warning, and the screen as it was saved.
• Z80 Registers: the main and alternate registers, IX, IY, I, R, the interrupt mode and flip-flops, and (.szx) the HALT state and WZ.
• ULA: the border, the position in the video frame (T-states since the interrupt), and which RAM bank the screen is shown from.
• Paging and Address Space at PC: $7FFD and $1FFD decoded, the program counter’s bank and offset, and what each 16K of the address space holds. The two buttons in the heading open the 64K the CPU sees as a memory dump or a disassembly, positioned at the program counter. A snapshot holds no ROM, so the ROM area is shown as zeros.
• AY-3-8912 (128K machines): the sixteen registers and what they mean for each channel.
• RAM Banks: every bank in one list, marked with where it is paged in, the screen bank, and PC or SP when the program counter or stack pointer points into it. Pop a bank out as a memory dump or a disassembly (listed at the address it is paged in, or $C000).
• File: the format’s own header fields; for .szx, every block of the file.

Loading, running and debugging

• Run (play icon): the machine runs from the snapshot’s state.
• Debug (debug icon): the machine starts in debug mode and stops at the snapshot’s program counter before executing that instruction, so the CPU, Memory and Disassembly panels show the saved state.

You can reach them from:

• the buttons in a snapshot’s document tab;
• the Explorer’s context menu on a snapshot file;
• the zx-snapshot command;
• File → Load ZX Spectrum Snapshot…, whatever machine is selected, and Load Snapshot… in the Emulator’s Machine menu while a ZX Spectrum is selected (both run the snapshot);
• dropping a snapshot file onto the Emulator window (it runs). Dropping a .tap or .tzx file inserts it as a tape; dropping a .z88 file opens it as a Z88 snapshot.

Which machine it runs on

The snapshot decides the machine; Klive switches to it when needed.

The warnings are there because the machine differs from the one the snapshot was saved on: Klive’s +2E and +3E use the +E ROMs, not Amstrad’s, so a program that calls into the ROM at a version-specific address may behave differently. Klive’s +2A and +3 models boot the Amstrad ROMs, so a +2A or +3 snapshot loaded on one of them gets no such warning. Klive also warns when the snapshot uses something it does not emulate — an AY chip on a 48K, Interface 1, an M.G.T. interface, TR-DOS, the Issue 2 keyboard, the alternate ULA timings, or a custom ROM.

Disks and tapes in .szx files

A .szx file can name the disks in the +3’s drives. Klive looks for each disk where the file says, then beside the snapshot, and inserts the ones it finds; a missing disk is reported. An embedded .tap or .tzx tape is put into the tape deck.

Things to know

• The 48K .sna stack quirk. A 48K .sna keeps the program counter on the stack, so the two bytes below the stack pointer hold it rather than what the program had there. This is how the format works in every emulator, and a few programs show it.
• No frame position in .sna and older .z80 files. The machine starts at the beginning of a video frame, just after the interrupt. Most programs do not notice; border and multicolour effects can glitch for one frame.
• The +3 disk controller restarts. Its state in the middle of a command cannot be restored, so a snapshot saved while the disk was busy may not resume that operation.
• Restart sets the machine up again from its configuration, so the snapshot is gone.

Saving a snapshot

Klive saves the ZX Spectrum 48K, 128K, +2A/+3 and +2E/+3E as a snapshot (a +2A or +3 model is written as a +2A or +3, a +2E/+3E as a +3e), whether the machine is running or paused. A running machine is paused for an instant and then runs on; a paused one stays paused. A stopped machine has nothing to save.

• File → Save ZX Spectrum Snapshot…, or Save Snapshot… in the Emulator’s Machine menu, asks for a file. The file type you pick (or the extension you type) decides the format.
• The zx-snapshot-save command saves to the file you name.

Save as .szx unless you need another format. It is the only format that keeps the whole state Klive captures; the other two lose a little, and Klive tells you exactly what after saving:

Only .szx keeps WZ (MEMPTR). It shows only in two undocumented flag bits after BIT n,(HL), so Klive does not list it as a loss. A 48K .sna pushes the program counter onto the stack in the file (never in the running machine), as every .sna writer does.

Some states cannot be saved in a format at all, because the file would load back as a different machine. Klive refuses them with the reason:

• a 48K .sna when the stack pointer is so low that the pushed program counter would land in ROM;
• a .sna of a +2E/+3E in special paging mode ($1FFD bit 0);
• a machine stopped between an instruction prefix (CB, DD, ED, FD) and its opcode, after a single step. Step once more and save again.