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Technical discussions about Freescale Microcontrollers: M68HC11. (Freescale Semiconductor is a Subsidiary of Motorola).
Text - bal_gill21 - Dec 15 13:12:00 2004
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hi, when I am writing ASII into memory ie fcc 'hello', i am having to check the very last memory location for 'o' to see when it has finished writing to the display. This value obviously changes when I add code (and I have to keep changing it). Is there a way of recognising the last letter so I don't have to keep changing the memory location every time my code changes? I could recognise that 'o' has been written but in some cases I have 2 blank spaces or two of the same letter. Thanks bal |
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Re: Text - Tony Papadimitriou - Dec 15 13:21:00 2004
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----- Original Message ----- From: "bal_gill21" <> To: < > when I am writing ASII into memory ie fcc 'hello', i am having to > check the very last memory location for 'o' to see when it has > finished writing to the display. This value obviously changes when I > add code (and I have to keep changing it). Is there a way of > recognising the last letter so I don't have to keep changing the > memory location every time my code changes? I could recognise > that 'o' has been written but in some cases I have 2 blank spaces or > two of the same letter. Yes, put zero at the end. fcc 'hello',0 or fcc 'hello' fcb 0 or (using my ASM11) fcs 'hello' It's also quicker to check for the end because LDA instructions also set the Z flag. So, there is no need to CMP against a specific value. > bal |
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Re: Text - Scott Grodevant - Dec 15 13:46:00 2004
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Hi all, If memory is critical and you have lots of messages, sometimes one sets the parity bit (bit 7) of the last character in the message, thus saving a byte of storage for each message. You then check for the character's value being above $7F. fcc 'hell','o'+$80 Scott Tony Papadimitriou <> wrote: ----- Original Message ----- From: "bal_gill21" To: > when I am writing ASII into memory ie fcc 'hello', i am having to > check the very last memory location for 'o' to see when it has > finished writing to the display. This value obviously changes when I > add code (and I have to keep changing it). Is there a way of > recognising the last letter so I don't have to keep changing the > memory location every time my code changes? I could recognise > that 'o' has been written but in some cases I have 2 blank spaces or > two of the same letter. Yes, put zero at the end. fcc 'hello',0 or fcc 'hello' fcb 0 or (using my ASM11) fcs 'hello' It's also quicker to check for the end because LDA instructions also set the Z flag. So, there is no need to CMP against a specific value. > bal Yahoo! Groups Links --------------------------------- |
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Re: Text - David Kelly - Dec 15 13:49:00 2004
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> > Yes, put zero at the end. > > fcc 'hello',0 ... > It's also quicker to check for the end because LDA instructions also set > the Z flag. So, there is no need to CMP against a specific value. In the heyday of Apple ][ and 6502 a common convention was to toggle the sign bit (test for N rather than Z) on the last character. Assemblers of the day had pseudo-opcodes for easily defining these strings. In an odd twist we usually set the N bit on most and cleared it on the last because straight ASCII displayed inverse. You will use more than one byte coding a test for the N bit then clearing/setting before display but if you have lots and lots of text strings a savings of one byte each may add up to something significant. The Pascal, VMS, and mainframe way was to prefix the string with a count. The Unix/C way of null termination has become dominant. Probably want to stick with the Unix/C way as most all string handling routines that you might borrow for your project will expect null terminated strings. |
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Re: Text - Tony Papadimitriou - Dec 15 14:14:00 2004
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This 'trick' was used with OS/9 (for the 6809). For me, it has two problems: 1. It is visually displeasing unless the assembler provides a pseudo-op to do it behind the scenes (e.g., fcs). 2. It only works for lower ASCII strings (i.e., useless for multilingual messaging). ----- Original Message ----- From: "Scott Grodevant" <> To: < Hi all, If memory is critical and you have lots of messages, sometimes one sets the parity bit (bit 7) of the last character in the message, thus saving a byte of storage for each message. You then check for the character's value being above $7F. fcc 'hell','o'+$80 Scott |
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Re: Text - Jim Peterson - Dec 16 8:42:00 2004
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I am not sure whether you realize it or not, but you now have a write text routine that (with some slight modification) could be made a subroutine to write any zero delimited string. See if you can push the address of the string onto the stack, JSR to the write string routine, whre it will pop the address off the stack (remember to preserve the JSR stack contents), write to the end of the string, then RTS back to the caller. A simpler approach might be to just assume D is pointing at the string on entry to the subroutine, but learning to manage the stack is an important step in advancing your programming skill. I deliberately did not include the specific details, it is important to make use of the reference manual for your MCU to keep this kind of sequence correct for any processor. Good luck, Jim bal_gill21 wrote: >Thanks for all the advice given, I have added a zero at the end and >it now saves me time by not having to change the compare address >every time the code changes. > >Bal > > >Yahoo! Groups Links |
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Re: Text - David Kelly - Dec 16 8:44:00 2004
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On Dec 16, 2004, at 4:15 AM, bal_gill21 wrote: > Thanks for all the advice given, I have added a zero at the end and > it now saves me time by not having to change the compare address > every time the code changes. *Manually* changing the compare address? Not something like this? hello_start fcc "Hello, World!" hello_end equ * hello_len equ hello_end-hello-start hello_len_too equ *-hello_start -- David Kelly N4HHE, ======================================================================== Whom computers would destroy, they must first drive mad. |
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Re: Text - Mark Schultz - Dec 16 10:29:00 2004
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--- In , "bal_gill21" <bal_gill21@y...> wrote: > > when I am writing ASII into memory ie fcc 'hello', i am having to > check the very last memory location for 'o' to see when it has > finished writing to the display. This value obviously changes when > I add code (and I have to keep changing it). Is there a way of > recognising the last letter so I don't have to keep changing the > memory location every time my code changes? I could recognise > that 'o' has been written but in some cases I have 2 blank spaces > or two of the same letter. One last note to add on this subject. As others have suggested, the typical way to terminate a string is to add a "zero terminator" to the end of it. You may encounter the term "ASCIIZ string" in some of the documentation or code you read; this term refers to the technique of terminating a string with a null/0 byte. If you use this technique, it is very easy to create a general- purpose subroutine to output a string (to the SCI or any other device). If you have a existing subroutine that outputs a single byte to, say, the SCI named "CharOut", the code below will allow you to output ANY zero-terminated string: ; Part of main program: ... ;(init code not shown) LDY #Hello ;Point to "Hello" text JSR Msg ;Output LDY #Goodbye ;Point to "Goodbye" text JSR Msg ;Output ... ;(rest of program) Hello FCS "Hello, world!",0 Goodbye FCS "Goodbye, cruel world.",0 ; Subroutine to output a single character CharOut ... ;(code not shown) ; Subroutine to output a ASCIIZ string ; Y <- Address of string to output Msg LDAA 0,Y ;Get char from string BEQ MsgX ;Exit if zero (end of str) JSR CharOut ;Output character INY ;Point to next character BRA Msg ;Continue until end of str MsgX RTS ;End of string, exit Note that I use the Y register as the string pointer - just LDY #String and call Msg to output it. You will need to create a subroutine named CharOut that handles single-character output to your device. If using the SCI for output, don't forget to include code to check if the transmit data register is empty. For SCI output, the CharOut subroutine will consist of 3 to 6 instructions, depending on how you choose to write it. Let me know if you need help with this part. -- Mark |
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