December 7, 2011 AT 11:08 am
Veronica – ROM Emulator
Follow up to previous post! Veronica – ROM Emulator @ Blondihacks. Quinn writes –
If you recall from last time, we got Veronica to perform a free run by NOP-ing her way through a phantom memory space.
Well, the next logical step is to get some real memory for her to run code from. I’m calling this a ROM Emulator, because it’s faking what would be the role of Read-Only Memory in a normal CPU startup sequence. The code and data will be entered into this memory by an external tool, and the CPU will not be able to write to it. In all other ways, however, it looks like regular memory to our girl.
A RAM chip by itself can’t do a whole lot. When powered up, it will be full of random junk. We need a method of data entry. Early computers usually had a big bank of toggle switches and blinking lights to handle this. If you’ve seen an old Altair or an old PDP-8, you know what I’m talking about. Hollywood has made sure we will always think Important Computers look like that. Anyways, the point of those switches and lights is to modify RAM directly, in order to give the CPU code to run immediately upon startup. It’s another one of those chicken-and-egg problems that plagued early computers. The CPU needs code in memory to run, but the CPU is usually what loads code into memory. You need a way to bootstrap that cycle.
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