2023-09-26 - Create note

2023-09-26.md

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+# 2023-09-26 — Dev notes
+
+## TODO
+
+- [ ] Review yesterday's notes on extended addressing
+- [ ] Add tape drive to mainframe
+- [ ] Add notes on re-arranging extended system docs
+
+## Start/Reset notes
+
+Idea/observation: if the processor begins execution at $FE (say),
+and instruction $00 is NOP, then after a few NOPs the instruction
+pointer will overflow and execution will continue at address $00.
+
+...which means that we can just pretend that it startts at $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.
+
+### 6502 Start/Reset
+
+https://www.masswerk.at/6502/6502_instruction_set.html:
+
+> Start/Reset Operations
+>
+> An active-low reset line allows to hold the processor in a known disabled
+state, while the system is initialized. As the reset line goes high, the
+processor performs a start sequence of 7 cycles, at the end of which the
+program counter (PC) is read from the address provided in the 16-bit reset
+vector at $FFFC (LB-HB). Then, at the eighth cycle, the processor transfers
+control by performing a JMP to the provided address.
+Any other initializations are left to the thus executed program. (Notably,
+instructions exist for the initialization and loading of all registers, but
+for the program counter, which is provided by the reset vector at $FFFC.)
+
+
+## Simulator memory management
+
+To support ROM, I think this is the best plan:
+
+- Create a new module that provides a "memory manager",
+  that contains the actual memory plus a set of annotations
+  indicating which parts are RAM and which are ROM
+- When instantiating the CPU, the simulator provides it with
+  an instance of the memory manager
+- 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...)