Version analysis of MT26 - Elite on the BBC Micro and NES

This code appears in the following versions (click to see it in the source code):

Disc (docked), 6502 Second Processor and Master

Code variations between these versions are shown below.

       Name: MT26
       Type: Subroutine
   Category: Text
    Summary: Fetch a line of text from the keyboard
  Deep dive: Extended text tokens

Code variation 1 of 7A variation in the comments only

This variation is blank in the Master version.

Docked, 6502SP
 If ESCAPE is pressed or a blank name is entered, then an empty string is
 returned.

 Returns:

   Y                    The size of the entered text, or 0 if none was entered

Code variation 2 of 7A variation in the comments only

This variation is blank in the Master version.

Docked, 6502SP
                        or if ESCAPE was pressed


   INWK+5               The entered text, terminated by a carriage return

   C flag               Set if ESCAPE was pressed


.MT26

Code variation 3 of 7Related to the Master version

When entering text in the Master version, the text that is typed is shown in magenta, while it is shown in white in the other versions.

This variation is blank in the Disc (docked) and 6502 Second Processor versions.

Master
 LDA COL                \ Store the current colour on the stack
 PHA

 LDA #RED               \ Switch to colour 2, which is magenta in the trade view
 STA COL

Code variation 4 of 7Related to Elite's use of the Tube

Tap on a block to expand it, and tap it again to revert.

Docked
Master
6502SP
 LDA #%10000001         \ Clear 6522 System VIA interrupt enable register IER
 STA VIA+&4E            \ (SHEILA &4E) bit 1 (i.e. enable the CA2 interrupt,
                        \ which comes from the keyboard)
 LDY #8                 \ Wait for 8/50 of a second (0.16 seconds)
 JSR DELAY

 JSR FLKB               \ Call FLKB to flush the keyboard buffer
 LDA #VIAE              \ Send a #VIAE %10000001 command to the I/O processor to
 JSR OSWRCH             \ clear 6522 System VIA interrupt enable register IER
 LDA #%10000001         \ (SHEILA &4E) bit 1 (i.e. enable the CA2 interrupt,
 JSR OSWRCH             \ which comes from the keyboard)

 LDY #8                 \ Wait for 8/50 of a second (0.16 seconds)
 JSR DELAY

Code variation 5 of 7Related to the Master version

The Master version contains its own custom routine for the extended token that asks for a line of text, while the other enhanced versions use the standard OSWORD command. The Master Compact version of that custom routine supports more characters, as it supports ADFS rather than DFS.

See below for more variations related to this code.

Tap on a block to expand it, and tap it again to revert.

Docked, 6502SP
Master
 JSR FLKB               \ Call FLKB to flush the keyboard buffer

 LDX #LO(RLINE)         \ Set (Y X) to point to the RLINE parameter block
 LDY #HI(RLINE)

 LDA #0                 \ Call OSWORD with A = 0 to read a line from the current
 JSR OSWORD             \ input stream (i.e. the keyboard)

 BCC P%+4               \ The C flag will be set if we pressed ESCAPE when
                        \ entering the name, otherwise it will be clear, so
                        \ skip the next instruction if ESCAPE is not pressed

 LDY #0                 \ ESCAPE was pressed, so set Y = 0 (as the OSWORD call
                        \ returns the length of the entered string in Y)
 LDY #0                 \ Set Y = 0 to hold the length of the text entered

.OSW0L

 JSR TT217              \ Scan the keyboard until a key is pressed, and return
                        \ the key's ASCII code in A (and X)

 CMP #13                \ If RETURN was pressed, jump to OSW03
 BEQ OSW03

 CMP #27                \ If ESCAPE was pressed, jump to OSW04
 BEQ OSW04

 CMP #127               \ If DELETE was pressed, jump to OSW05
 BEQ OSW05

IF _COMPACT

 PHX                    \ Store X on the stack so we can retrieve it after the
                        \ call to SHIFT

 PHA                    \ Store the number of the key being pressed on the stack

 JSR SHIFT              \ If SHIFT is not being pressed, jump to noshift to skip
 BPL noshift            \ the following

 PLA                    \ SHIFT is being pressed, so fetch the number of the key
                        \ being pressed from the stack

 CMP #'@'               \ If A >= ASCII "@", then we are pressing a letter key,
 BCS P%+4               \ so skip the following instruction

 EOR #%00010000         \ We are pressing SHIFT and a number key, so flip bit 4
                        \ of the key number, which flips the letter between the
                        \ ASCII code of the number being pressed and the ASCII
                        \ code of the number being pressed when SHIFT is being
                        \ held down (so SHIFT-1 will enter !, SHIFT-2 will enter
                        \ ", and so on)

 PHA                    \ Push the updated key number onto the stack

.noshift

 PLA                    \ Retrieve the values of X and A we stored on the stack
 PLX                    \ above

ENDIF

 CPY RLINE+2            \ If Y >= RLINE+2 (the maximum line length from the
 BCS OSW01              \ OSWORD configuration block at RLINE), then jump to
                        \ OSW01 to give an error beep as we have reached the
                        \ character limit

 CMP RLINE+3            \ If the key pressed is less than the character in
 BCC OSW01              \ RLINE+3 (the lowest allowed character from the OSWORD
                        \ configuration block at RLINE), then jump to OSW01
                        \ to give an error beep as the key pressed is out of
                        \ range

 CMP RLINE+4            \ If the key pressed is greater than or equal to the
 BCS OSW01              \ character in RLINE+4 (the highest allowed character
                        \ from the OSWORD configuration block at RLINE), then
                        \ jump to OSW01 to give an error beep as the key
                        \ pressed is out of range

 STA INWK+5,Y           \ Store the key's ASCII code in the Y-th byte of INWK+5

 INY                    \ Increment Y to point to the next free byte in INWK+5

 EQUB &2C               \ Skip the next instruction by turning it into
                        \ &2C &A9 &07, or BIT &07A9, which does nothing apart
                        \ from affect the flags

.OSW01

 LDA #7                 \ Set A to the beep character, so the next instruction
                        \ makes a system beep

.OSW06

 JSR CHPR               \ Print the character in A (and clear the C flag)

 BCC OSW0L              \ Loop back to OSW0L to fetch another key press (this
                        \ BCC is effectively a JMP as CHPR clears the C flag)

.OSW03

 STA INWK+5,Y           \ Store the return character in the Y-th byte of INWK+5

 LDA #12                \ Print a newline
 JSR CHPR

 EQUB &24               \ Skip the next instruction by turning it into &24 &38,
                        \ or BIT &0038, which does nothing apart from affect the
                        \ flags

.OSW04

 SEC                    \ Set the C flag as ESCAPE was pressed

 PLA                    \ Restore the original colour from the stack and set it
 STA COL                \ as the current colour

 RTS                    \ Return from the subroutine

.OSW05

 TYA                    \ If the length of the line so far in Y is 0, then we
 BEQ OSW01              \ just pressed DELETE on an empty line, so jump to
                        \ OSW01 give an error beep

 DEY                    \ Otherwise we want to delete a character, so decrement
                        \ the length of the line so far in Y

 LDA #127               \ Set A = 127 and jump back to OSW06 to print the
 BNE OSW06              \ character in A (i.e. the DELETE character) and listen
                        \ for the next key press

Code variation 6 of 7Related to Elite's use of the Tube

This variation is blank in the Master version.

Tap on a block to expand it, and tap it again to revert.

Docked
6502SP
 LDA #%00000001         \ Set 6522 System VIA interrupt enable register IER
 STA VIA+&4E            \ (SHEILA &4E) bit 1 (i.e. disable the CA2 interrupt,
                        \ which comes from the keyboard)
 LDA #VIAE              \ Send a #VIAE %00000001 command to the I/O processor to
 JSR OSWRCH             \ set 6522 System VIA interrupt enable register IER
 LDA #%00000001         \ (SHEILA &4E) bit 1 (i.e. disable the CA2 interrupt,
 JSR OSWRCH             \ which comes from the keyboard)

Code variation 7 of 7Related to the Master version

See variation 5 above for details.

This variation is blank in the Master version.

Docked, 6502SP
 JMP FEED               \ Jump to FEED to print a newline, returning from the
                        \ subroutine using a tail call