Interrupts unclearly explained
When I read day 23 back in the day, I was very confused. Let me sum up the causes of my confusion.
In mode 0, only external hardware peripherals can generate interrupts. The ISR is a single byte sent by the peripheral that the CPU executes as an opcode. The calculator cannot use mode 0.
At a frequency of about 140Hz or less, the CPU executes a RST 28H instruction. The TI-83 Plus uses this interrupt to detect keys, blink the cursor, check the linkport, etc.
The system interrupt uses SP (naturally) and IY, so if you want to use these registers for other purposes you need to disable interrupts (or just not process the system interrupt).
The most fun. In mode 2, the CPU can theoretically jump to any address in memory. This is the interrupt mode we are interested in.
So first, the description of mode 1 is just wrong. 1. It executes RST 38H, not RST 28H, and 2. Using the term "frequency" made me think that it actually lowered the frequency of the CPU to 140 Hz. It should just say that it executes RST 38H on an interrupt, and the 140 Hz interrupt source should be covered separately, because it applies to the other interrupt modes as well. Also, mode 0 interrupts can equally be emulated by inserting the instruction you want.
A bit further down:
The meanings of bits 2 and 3 are not known.
They are known.
And on the same line:
To return, output
This is wrong, it's