darknezz19 wrote:
Alright understood, thanks for that info Memblers. This is just trying to interface a flash chip to act as a ROM in a non intrusive way MaarioS. As for the A20, yeah it's weird, but it's really /CE.
Earlier today I realized that the EXT pinout is for the PC Engine, and the TG16 pins differ in that the d0-d7 lines are reversed. I swapped those and double checked all my wiring, but it's still giving me trouble. It's probably something simple I'm just forgetting and will come back to it in a day or two with a fresh slate.
A20 is used as /CE for hucards. The upper address range was supposedly reserved for rear bus addons, so A20 was a cheap way to handle this. But you don't have to. There's plenty of open bus areas that can be access from both the hucard port and the back plane, equally. The Arcade Card (both versions), maps some hardware registers into the $1FFA00-1FFAFF range (from the hucard port), as well as mapping two byte registers mirrored into 8k banks ($80000-87FFF). There's also a hucard detect 'pin', which is used to enable/disable lower mapped ram/rom/etc to the lower range depending if a hucard is present (this can be controlled via the hucard port; the later game express cards do this to access superCD ram, but still use its own rom - IIRC), though the reaction of that pin status is directly handled by whatever rear bus addon.
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