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by Pxtl
3814 days ago
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That's neat. But did that only work then with games that had writable storage media in the cartridge? I know that was rare on NES games. Or was there secondary memory besides the RAM and disk that allowed for data to be passed between resets? |
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In later consoles that had their own MMUs, like the PSX, this wasn't a full hardware feature anymore, but rather a simulated convention. You'd "reboot" by dropping all your virtual-memory mappings except one, then asking the disk to async-fill some buffers from the binary you wanted to launch and then mapping those pages and jumping into them on completion. (Basically like unloading one DLL and then loading a different one, except you're also forcefully dropping all the heap allocations the old DLL made when you unload it.)
In both the hard and soft implementations, the "volatile SRAM" page could be thought of as basically a writeback cache for the state in the actual battery-backed SRAM. You wouldn't want to do individual byte-level writes to SRAM (writes to SRAM were slowwww), so when the game booted, you'd mirror SRAM to your state-page, and then update the state-page whenever you had something you wanted to persist—finally dumping it out to battery-backed SRAM when the player hit "Save". Basically, most games were "auto-saving" from the beginning—but they were auto-saving to volatile memory.
But even games that had true "auto-saving", like Yoshi's Island, still kept an "SRAM buffer" like this; the write-to-SRAM event was just managed as a sequence of smaller bursts of memory-mapped IO done by modules that sat there playing music+animations and not doing any logic, like YI's "field/title card" submodule when re-entered from a loss-of-life event, or its "field/score card" submodule entered by completing a stage. If there is ever what seems to be a "pointlessly long" animation in an auto-saving game of that era near a state-transition, it's probably by design, to cover for an SRAM cache-flush. (The fact that flashy "rewarding" animations turned out to also be good game design, favored by slot-machine and casual-game designers the world over, is mostly coincidence.)
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ETA: when you had neither kind of SRAM, but still wanted to preserve some state across a reboot, what could you do? Well, write it to video memory, of course!
On a system with a PPU, the PPU owned the VRAM; it wasn't the CPU's job to reset it, but rather the PPU's. On a system with only a framebuffer, nothing owned the framebuffer (or rather, the framebuffer was an inherited abstraction from the character buffers of teletypes: the "client" owned the framebuffer, so it was up to the "client" to erase it. Restarting a mainframe shouldn't forcibly clear all its connected teletypes; disconnecting from an SSH session shouldn't forcibly clear your terminal, but rather optionally clear your terminal as a way your TTY driver is set to respond to the signal; etc.)
Either way, if you could get your data into VRAM generally or the framebuffer specifically, you could very likely read it back after reboot.
VRAM is also where many "online" development suites—the BASICs and Pascals of the time—expected you to write exception traces. Rather than trying to "break into" a debugger (i.e. cram a debugger into the same address space as your software), you'd simply have your trap-handler persist the stack-trace to VRAM, switch your tape drive over to the devtools disk, and reboot. The "monitor" would load, notice that there's a stack-trace in VRAM, parse it, read the pages it mentions from your program (now on the slave tape) and display them.