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Elite on the BBC Micro

Market: gnum [Electron version]

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 Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * EQSHP calls gnum * TT219 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 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) 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" 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" 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