Printing text
=============
&F6/7 are allowed to be used as they are only otherwise used when calling
OSRDRM and when ROMFS makes sideways ROM calls. As you can't *SPOOL text to
ROMFS as it is a read-only filing system, printing text won't result in a
ROMFS filing system call that uses &F6/7.

  \ Print inline string
  \ =====================
  \ Prints any length string
  \ Preserves X,Y
  \ Corrupts  A
  \ Size      34 bytes
  \
  \ Called with:
  \ JSR PrText
  \ EQUS "text message"
  \ EQUB 0
  .PrText
  PLA:STA &F6:PLA:STA &F7   :\ &F6/7=>embedded string
  TYA:PHA                   :\ Save Y
  JMP PrTextSkip            :\ Step to first byte
  .PrTextLp
  LDY #0:LDA (&F6),Y        :\ Get character
  BEQ PrTextSkip            :\ Not zero, skip printing
  JSR OSASCI                :\ Print character
  .PrTextSkip
  INC &F6:BNE PrTextNext    :\ Increment address
  INC &F7
  .PrTextNext
  TAY:BNE PrTextLp          :\ Not zero, loop for next
  PLA:TAY                   :\ Restore Y
  JMP (&00F6)

  \ Print inline text
  \ =================
  \ Prints up to 255 characters
  \ Preserves X
  \ Corrupts  A,Y
  \ Size      31 bytes
  \
  \ Called with:
  \ JSR  PrText
  \ EQUS "text message"
  \ EQUB 0
  .PrText
  PLA:STA &F6:PLA:STA &F7   :\ &F6/7=>embedded string
  LDY #1                    :\ Offset to string
  .PrTextLp
  LDA (&F6),Y:BEQ PrTextEnd :\ Get character, exit if &00
  JSR OSASCI                :\ Print character
  INY:BNE PrTextLp          :\ Point to next, loop back for next
  .PrTextEnd
  CLC:TYA:ADC &F6:TAY       :\ Update pointer and stack it
  LDA #0:ADC &F7:PHA
  TYA:PHA:RTS               :\ Return to code after string

  \ Print inline string
  \ =====================
  \ Prints any length string
  \ Preserves X
  \ Returns   A=&00, Y=&00, EQ
  \ Size      29 bytes
  \
  \ Called with:
  \ JSR PrText
  \ EQUS "text message"
  \ EQUB 0
  .PrText
  PLA:STA &F6:PLA:STA &F7   :\ &F6/7=>embedded string
  BNE PrTextSkip            :\ Step to first byte
  .PrTextLp
  LDY #0:LDA (&F6),Y        :\ Get character
  BEQ PrTextSkip            :\ Not zero, skip printing
  JSR OSASCI                :\ Print character
  .PrTextSkip
  INC &F6:BNE PrTextNext    :\ Increment address
  INC &F7
  .PrTextNext
  TAY:BNE PrTextLp          :\ Not zero, loop for next
  JMP (&00F6)


Generate error from sideways ROM
================================
When running code in a service ROM you can't generate an error with BRK as
the current language will be in another ROM and won't be able to read it.
The standard method is to copy the error to the bottom of the stack and
execute it there.

  \ Generate inline error
  \ =====================
  \ Called with:
  \ JSR MkError
  \ EQUB errornumber           :\ must not be &00
  \ EQUS "error message"
  \ EQUB 0
  .MkError
  PLA:STA &FD:PLA:STA &FE      :\ Pop address after JSR
  LDY #0                       :\ Index into the error block
  .MkErrorLp
  INY:LDA (&FD),Y:STA &100,Y   :\ Copy error block to stack
  BNE MkErrorLp                :\ Loop until terminating &00 byte
  STA &100:JMP &100            :\ Store &00 as BRK and jump to it

&FD/E are allowed to be used as they are the error pointer and will be
overwritten immediately after the JMP &100 when the BRK handler sets them up.








&F5 use to hold a ROM number.