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Elite on the BBC Micro

Market: gnum [Elite-A, Encyclopedia]

Name: gnum [Show more] Type: Subroutine Category: Market Summary: Get a number from the keyboard
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * menu calls gnum

Get a number from the keyboard, up to the maximum number in QQ25, for the buying and selling of cargo and equipment. Pressing "Y" will return the maximum number (i.e. buy/sell all items), while pressing "N" will abort the sale and return a 0. Pressing a key with an ASCII code less than ASCII "0" will return a 0 in A (so that includes pressing Space or Return), while pressing a key with an ASCII code greater than ASCII "9" will jump to the Inventory screen (so that includes all letters and most punctuation). Arguments: QQ25 The maximum number allowed Returns: A The number entered R Also contains the number entered C flag Set if the number is too large (> QQ25), clear otherwise
.gnum LDX #0 \ We will build the number entered in R, so initialise STX R \ it with 0 LDX #12 \ We will check for up to 12 key presses, so set a STX T1 \ counter in T1 .TT223 JSR TT217 \ Scan the keyboard until a key is pressed, and return \ the key's ASCII code in A (and X) LDX R \ If R is non-zero then skip to NWDAV2, as we are BNE NWDAV2 \ already building a number \ --- Mod: Original Acornsoft code removed: -----------> \ CMP #'y' \ If "Y" was pressed, jump to NWDAV1 to return the \ BEQ NWDAV1 \ maximum number allowed (i.e. buy/sell the whole stock) \ CMP #'n' \ If "N" was pressed, jump to NWDAV3 to return from the \ BEQ NWDAV3 \ subroutine with a result of 0 (i.e. abort transaction) \ --- End of removed code -----------------------------> .NWDAV2 STA Q \ Store the key pressed in Q 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) BCC OUT \ If A < 0, jump to OUT to return from the subroutine \ with a result of 0, as the key pressed was not a \ number or letter and is less than ASCII "0" \ --- Mod: Original Acornsoft code removed: -----------> \ CMP #10 \ If A >= 10, jump to BAY2 to display the Inventory \ BCS BAY2 \ screen, as the key pressed was a letter or other \ \ non-digit and is greater than ASCII "9" \ --- And replaced by: --------------------------------> CMP #10 \ If A >= 10, jump to buy_invnt to decide which screen BCS buy_invnt \ to display, as the key pressed was a letter or other \ non-digit and is greater than ASCII "9" (so it could \ be a red function key, for example) \ --- End of replacement ------------------------------> STA S \ Store the numeric value of the key pressed in S LDA R \ Fetch the result so far into A CMP #26 \ If A >= 26, where A is the number entered so far, then BCS OUT \ adding a further digit will make it bigger than 256, \ so jump to OUT to return from the subroutine with the \ result in R (i.e. ignore the last key press) ASL A \ Set A = (A * 2) + (A * 8) = A * 10 STA T ASL A ASL A ADC T ADC S \ Add the pressed digit to A and store in R, so R now STA R \ contains its previous value with the new key press \ tacked onto the end CMP QQ25 \ If the result in R = the maximum allowed in QQ25, jump BEQ TT226 \ to TT226 to print the key press and keep looping (the \ BEQ is needed because the BCS below would jump to OUT \ if R >= QQ25, which we don't want) BCS OUT \ If the result in R > QQ25, jump to OUT to return from \ the subroutine with the result in R .TT226 LDA Q \ Print the character in Q (i.e. the key that was JSR TT26 \ pressed, as we stored the ASCII value in Q earlier) DEC T1 \ Decrement the loop counter BNE TT223 \ Loop back to TT223 until we have checked for 12 digits .OUT LDA R \ Set A to the result we have been building in R RTS \ Return from the subroutine \ --- Mod: Original Acornsoft code removed: -----------> \.NWDAV1 \ \ If we get here then "Y" was pressed, so we return the \ \ maximum number allowed, which is in QQ25 \ JSR TT26 \ Print the character for the key that was pressed \ LDA QQ25 \ Set R = QQ25, so we return the maximum value allowed \ STA R \ RTS \ Return from the subroutine \.NWDAV3 \ \ If we get here then "N" was pressed, so we return 0 \ JSR TT26 \ Print the character for the key that was pressed \ LDA #0 \ Set R = 0, so we return 0 \ STA R \ RTS \ Return from the subroutine \ --- End of removed code ----------------------------->