^ma 1 56 ^pl 0 ^ju
^sk0 PATCH FOR ESMLDL-OS 
^sk0 ------------------- 
^coEquipment required  HP-41 (any model) 
^co                    RAM and Eprom box. 
^co                    ESMLDL-OS 
^ju^sk1A bug was reported at the London meeting with ESMLDL-OS 
version 7B.  With the OS attached, if a PSE was executed in a 
FOCal program, the calculator would hang.  
Investigation found the following code in the OS:- 
^sk1 pse  xFF4 3E0  RTN 
^sk1xFF4 is the first of the interrupt locations at the 
end of the ROM and is checked whenever a PSE is executed.  
If the location is non-zero a jump is performed to xFF4.  
This is normally a JNC to the interrupt code.  
The correct way to exit an interrupt routine is to NC GO RMCK10 or 
RMCK15.  The return stack contains no valid addresses 
and in any case appears to be 0.  
Thus when a PSE is executed, the calculator checks xFF4 
and finding it non-zero executes a RTN to 0.  
As the display is on this puts the calculator into light sleep.  
^sk1To solve the problem it is necessary to prevent the calculator 
checking the OS ROM.  
When the calculator is scanning the ROMs the C register is formatted 
as follows:  
^sk1xx:xr:rr:rn:FF:4x:xx 
^sk1with PTR P=6.  
^sk1rrrr is the address that ROMCHK will return to after polling 
all the ROMs.  
n is the number of the ROM being entered.  
Two code segments can be used to prevent the OS ROM being checked.  
The first one restarts the checking of 
interrupt locations with the ROM after the OS ROM.  
^sk1
^coskip  xFF1 350  LD@R D  Where the OS  
^co      xFF2 3CD 09E  NC GO RMCK10  Rom is in page  
^copse   xFF4 3EB  JNC skip  D & x ( D  
^ju^sk1This makes n equal to the page address of the OS using the fact that P=6.  
On return to ROMCHK, n is incremented to create the address of the next ROM to 
be checked, thus starting at the page after the OS ROM. 
Obviously if this code is not in the page immediately before the 
OS ROM some ROMs will not be scanned.  
This is not as serious as it sounds as not 
many ROMs use this entry point.  
The other code segment is used if the OS is in page F 
or if there is no need to scan other ROMs.  
It kills the poll stone dead and is one word shorter.  
^sk1
^cokilpol  xFF2 07C  RCR 4  Get ROMCHK rtn  
^co        xFF3 1E0  GOTO ADR  and jump to it  
^copse     xFF4 3F3  JNC kilpol  
^ju^sk1ROMCHK is located at 27E6 (23746oct) and 
begins on page 270 of the VASM listings.  
For another interrupt routine see my auto loop power down routine.  
^sk1Rabin Ezra [#19H]