Re: Running and returning
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Bob Kircher wrote in,
"If I were to write a program to run on the Z80MC in high memory (not?
under CPM) what is the preferred way to return to the monitor?? I had?
been using JP 0042H ( JP VMAIN_MENU ) but I recently discovered I can?
not change any registers after I get back to the monitor.? I can also?
return by jumping to location zero but I'd better be quick about hitting?
the M key else I accidentally boot CP/M."
Great question, worth posting to the forum.
I'm not 100% sure what I wrote and debugged but having a quick look at the source code it appears only execution to 0000 will save the registers.
As such, when executing the G command, the monitor places 0000 on stack as the return address.? So your code needs to only RET to get back to the monitor.
LD HL,0 ;10 Set Default return address as 0000 (Restart/Save Registers) PUSH HL ;11? ?& Put on stack
Now, it should know/resolve the restart type and not prompt you to press M for monitor... but I'm not sure if that works... certainly won't be the only bug in the system.? If you really want, you can fake the M key by entering it into the buffer yourself before returning to 0000.
Here's a snipit of the code that saves the bit banged character to the buffer.??
IRXD_SAVE LD? A,H ;4 ;Fetch received byte LD? HL,(RXBHEAD) ;16 ;Advance Head Ptr of RX Buffer, Head = Head + 1 INC L ;4 LD? (RXBHEAD),HL ;16 LD? (HL),A ;7 ;Save Received byte into RX Buffer
Actually, instead of faking the M key, you can set the SOFT_RST_FLAG value (I think the firmware might already be doing this.. but I cant' figure out where).
When the firmware boots up, it runs code to check this flag as follows. If you set this flag to 0xD1 (single step return), it should return to the monitor and automatically dump the registers.
;Determine why the CPU is executing A RESET. ;? -Power on (RAM signature will be wrong) ;? -F-0 Reset Switch (one of the switches will still be pressed) ;? -RST 0 (look for C7 at previous location given by stack) ;? -External /RESET OR User program branches to 0000 ; ;RC_TYPE ;0 COLD RESET (NO RAM SIGNATURE) ;1 F-E Soft Reset (SOFT_RST_FLAG = FE) ;2 Single Step (SOFT_RST_FLAG = D1) ;3 <Ctrl>-C (SOFT_RST_FLAG = CC) ;4 HALT (SOFT_RST_FLAG = 76) ;5 F-0 (F or 0 key on FP still down) ;6 RST0 ( M(PC-1) = C7) ;7 HARD RESET (Default if SOFT_RST_FLAG not set)
Hopefully the RET should work fine, but if not, it's definitely one of the things I will be addressing in the next version of firmware.
Regards, Josh
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