2023-09-26 - Update

2023-09-26.md

@@ -16,6 +16,9 @@ pointer will overflow and execution will continue at address $00.
 but also have the option to add ROM at the top of memory, and have
 a JMP at the top that redirects execution to somewhere else in ROM.
 
+(This overflow behaviour is normal; see 
+https://stackoverflow.com/questions/26139662/program-counter-overflow)
+
 ### 6502 Start/Reset
 
 https://www.masswerk.at/6502/6502_instruction_set.html:
@@ -45,4 +48,23 @@ To support ROM, I think this is the best plan:
 - The memory manager has a `.write(startingAddress, bytes)` method
   — and it just does nothing when the address is in ROM
 - (The memory manager might also need to provide some way of
-  attaching memory-mapped peripherals...)
+  attaching memory-mapped peripherals...)
+
+
+## Changes to Microprocessor Trainer keypad IO ?
+
+I think the DMA plan for the display is fine,
+because probably the display controller can just tap into
+the same address and read-signal lines as the main memory.
+
+...but the keypad input is maybe trickier to do by DMA?
+It could be done by cycle stealing.
+(Or it could be done by halting the processor, writing to memory,
+and then resuming... but that's a bad plan for the keypad.)
+
+It seems like it would be simple enough, though, to use the
+new IO system I've been planning for the mainframe version--
+an `INP` could just enable the keypad to dump its current state
+directly onto the bus.
+
+- [ ] TODO: think a bit about how the INP/OUT commands would work electrically