Version analysis of qv - Elite on the 6502

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

Cassette, Disc (docked), 6502 Second Processor, Master and Electron

Code variations between these versions are shown below.

       Name: qv
       Type: Subroutine
   Category: Equipment
    Summary: Print a menu of the four space views, for buying lasers


 Print a menu in the bottom-middle of the screen, at row 16, column 12, that
 lists the four available space views, like this:

                 0 Front
                 1 Rear
                 2 Left
                 3 Right

 Also print a "View ?" prompt and ask for a view number. The menu is shown
 when we choose to buy a new laser in the Equip Ship screen.


 Returns:

   X                    The chosen view number (0-3)


.qv

Code variation 1 of 6Related to a standard feature

When buying a laser in the cassette version, the menu of available views is always shown below the equipment list. In the other versions, the list of equipment in systems with tech level 8 and above is too long to squeeze in the menu (due to the extra lasers you can buy in these versions), so when buying lasers in these systems, the whole screen is cleared and the menu is shown in the middle of the screen.

This variation is blank in the Cassette and Electron versions.

Docked, 6502SP, Master
 LDA tek                \ If the current system's tech level is less than 8,
 CMP #8                 \ skip the next two instructions, otherwise we clear the
 BCC P%+7               \ screen to prevent the view menu from clashing with the
                        \ longer equipment menu available in higher tech systems

 LDA #32                \ Clear the top part of the screen, draw a border box,
 JSR TT66               \ and set the current view type in QQ11 to 32 (Equip
                        \ Ship screen)

Code variation 2 of 6Related to Elite's use of the Tube

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

Cassette, Docked, Electron
Master
6502SP
 LDY #16                \ Move the text cursor to row 16, and at the same time
 STY YC                 \ set Y to a counter going from 16 to 19 in the loop
                        \ below
 LDA #16                \ Move the text cursor to row 16, and at the same time
 TAY                    \ set Y to a counter going from 16 to 19 in the loop
 STA YC                 \ below
 LDA #16                \ Move the text cursor to row 16, and at the same time
 TAY                    \ set Y to a counter going from 16 to 19 in the loop
 JSR DOYC               \ below

.qv1

Code variation 3 of 6Related to Elite's use of the Tube

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

Cassette, Docked, Electron
Master
6502SP
 LDX #12                \ Move the text cursor to column 12
 STX XC
 LDA #12                \ Move the text cursor to column 12
 STA XC
 LDA #12                \ Move the text cursor to column 12
 JSR DOXC

 TYA                    \ Transfer the counter value from Y to A

 CLC                    \ Print ASCII character "0" - 16 + A, so as A goes from
 ADC #'0'-16            \ 16 to 19, this prints "0" through "3" followed by a
 JSR spc                \ space

 LDA YC                 \ Print recursive text token 80 + YC, so as YC goes from
 CLC                    \ 16 to 19, this prints "FRONT", "REAR", "LEFT" and
 ADC #80                \ "RIGHT"
 JSR TT27

Code variation 4 of 6Related to Elite's use of the Tube

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

Cassette, Docked, Master, Electron
6502SP
 INC YC                 \ Move the text cursor down a row, and increment the
                        \ counter in YC at the same time
 JSR INCYC              \ Move the text cursor down a row, and increment the
                        \ counter in YC at the same time

 LDY YC                 \ Update Y with the incremented counter in YC

 CPY #20                \ If Y < 20 then loop back up to qv1 to print the next
 BCC qv1                \ view in the menu

Code variation 5 of 6A variation in the labels only

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

Cassette, Electron
.qv3

 JSR CLYNS              \ Clear the bottom three text rows of the upper screen,
                        \ and move the text cursor to the first cleared row

.qv2

 LDA #175               \ Print recursive text token 15 ("VIEW ") followed by
 JSR prq                \ a question mark

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

 SEC                    \ Subtract ASCII "0" from the key pressed, to leave the
 SBC #'0'               \ numeric value of the key in A (if it was a number key)

Code variation 6 of 6Minor and very low-impact

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

Cassette, Electron
Docked, 6502SP, Master
 CMP #4                 \ If the number entered in A >= 4, then it is not a
 BCS qv3                \ valid view number, so jump back to qv3 to try again
 CMP #4                 \ If the number entered in A < 4, then it is a valid
 BCC qv3                \ view number, so jump down to qv3 as we are done

 JSR CLYNS              \ Otherwise we didn't get a valid view number, so clear
                        \ the bottom three text rows of the upper screen, and
                        \ move the text cursor to column 1 on row 21

 JMP qv2                \ Jump back to qv2 to try again

.qv3

 TAX                    \ We have a valid view number, so transfer it to X

 RTS                    \ Return from the subroutine