Hi,
I've grown very interested in the Mister project & interested in learn more in particular about the NES core. I'm trying to follow the process in how ROMs are loaded, from the user interface in Linux down to it being accessed from the CPU/PPU.
This is my understand so far... It looks like roms can be stored any place by a user of the Mister Project and load this via a UI. I don't have a Mister myself =/ not yet. The ROM is loaded from
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sys_top.v -> nes.sv -> hps_io -> gameLoader
in gameloader there are 4 file types:
wire type_bios = filetype[0];
wire type_nes = filetype[1];
wire type_fds = filetype[2];
wire type_nsf = filetype[3];
I'm not sure what bios means? But I believe nes is the ROM header, fds relates to the nes disk system, & nsf to a header related to sound
gameLoader -> nes.v -> cart_top -> (to many module instances, once for each mapper)
in generic.sv, looking at mapper28 as an example
I'm trying to understand what the following signals are and how the relate to mapping the ROM/RAM seen by the NES
reg [6:0] a53prg; // output PRG ROM (A14-A20 on ROM)
reg [1:0] a53chr; // output CHR RAM (A13-A14 on RAM)
reg [3:0] inner; // "inner" bank at 01h
reg [5:0] mode; // mode register at 80h
reg [5:0] outer; // "outer" bank at 81h
reg [2:0] selreg; // selector register
reg [3:0] security; // selector register
Following this I believe the ROM is ultimately loaded into SDRAM. I think the mapper is designed such that
when the NES needs to access either ROM/RAM it is routed to via a mapper to locations in SDRAM that
don't change, regardless of the mapper used.