On the hex keyboard there is an SST switch.
In the ON state one can single step to a program, by pressing GO. After the instruction a the current PC is executed the display lights up again and the keys are active.
All registers are saved in zeropage ready for inspection. Pressing GO again resumes the next instruction with the saved CPU registers.
To use this the NMI vector must be initialized by the user at 17FA and 17FB to 00 and 1C
The instructions how to use this are in Paragraph 5.5 of the User Manual. There it is only described for the LED display and Keyboard mode, but it also works at the TTY interface.
Note that you cannot single step the monitor ROM routines!
How does that work?
The hardware for SST operation is shown in the next figure
The output of the NAND port 26 is connected to NMI when the SST switch is ON. Input 5 is connected to the SYNC signal of the 6502. SYNC goes high for the first byte of an opcode fetch.
So when an opcode fetch occurs NMI is activated and when the current instruction is finished the NMI handler is invoked.
This means a single step of the CPU!
The other input of the NAND, pin 4, is connected to K7, the chip select line for the KIM-1 monitor ROM in the 6530-002. So NMI is blocked when an opcode is fetched from the KIM-1 monitor, and the keyboard and LED can be active.
NMI, BRK and IRQ handling
The 6502 NMI interrupt is available as ST key on the hex keyboard via a NE556 debounce circuit.
The IRQ line is unconnected (quiet via a resistor pull)
The 6502 vectors are:
1213 1FFA 1C 1C NMIENT .WORD NMIT 1214 1FFC 22 1C RSTENT .WORD RST 1215 1FFE 1F 1C IRQENT .WORD IRQT
0602 1C1C ; 0603 1C1C 6C FA 17 NMIT JMP (NMIV) ; NON-MASKABLE INTERRUPT TRAP 0604 1C1F 6C FE 17 IRQT JMP (IRQV) ; INTERRUPT TRAP
These entries are not initialized by the KIM-1 monitor, that is up to the user.
The interrupt handler, saves all CPU registers at the relevant zeropage locations and restarts the KIM- monitor.
0587 1C00 85 F3 SAVE STA ACC ; KIM ENTRY VIA STOP (NMI) 0588 1C02 68 PLA ; OR BRK (IRQ) 0589 1C03 85 F1 STA PREG 0590 1C05 68 PLA ; KIM ENTRY VIA JSR (A LOST) 0591 1C06 85 EF STA PCL 0592 1C08 85 FA STA POINTL 0593 1C0A 68 PLA 0594 1C0B 85 F0 STA PCH 0595 1C0D 85 FB STA POINTH 0596 1C0F 84 F4 STY YREG 0597 1C11 86 F5 STX XREG 0598 1C13 BA TSX 0599 1C14 86 F2 STX SPUSER 0600 1C16 20 88 1E JSR INITS 0601 1C19 4C 4F 1C JMP START
The JMP START resumes the KIM-1 monitor, so in SST operation the suer has control over the system and can inspect memory like saved registers.
Now read about GOEXEC to see how the next instruction in SST mode gets its registers back.
To use the NMI line one must fill the NMI vector at 17FA. For SST and ST key this has to hold $1C00, the SAVE routine.
To use a hardware IRQ or a BRK instruction the IRQ vector at 17FE has to be filled, SAVE $1C00 is the recommended vector.