Skip to navigation

BBC Micro Elite

Dashboard: DILX (6502SP version)

Name: DILX [View in context] Type: Subroutine Category: Dashboard Summary: Update a bar-based indicator on the dashboard Deep dive: The dashboard indicators
The range of values shown on the indicator depends on which entry point is called. For the default entry point of DILX, the range is 0-255 (as the value passed in A is one byte). The other entry points are shown below. Arguments: A The value to be shown on the indicator (so the larger the value, the longer the bar) T1 The threshold at which we change the indicator's colour from the low value colour to the high value colour. The threshold is in pixels, so it should have a value from 0-16, as each bar indicator is 16 pixels wide K The colour to use when A is a high value, as a 2-pixel mode 2 character row byte K+1 The colour to use when A is a low value, as a 2-pixel mode 2 character row byte SC(1 0) The screen address of the first character block in the indicator Other entry points: DILX+2 The range of the indicator is 0-64 (for the fuel indicator) DIL-1 The range of the indicator is 0-32 (for the speed indicator) DIL The range of the indicator is 0-16 (for the energy banks)
.DILX LSR A \ If we call DILX, we set A = A / 16, so A is 0-15 LSR A LSR A \ If we call DILX+2, we set A = A / 4, so A is 0-15 LSR A \ If we call DIL-1, we set A = A / 2, so A is 0-15 .DIL \ If we call DIL, we leave A alone, so A is 0-15 STA Q \ Store the indicator value in Q, now reduced to 0-15, \ which is the length of the indicator to draw in pixels LDX #&FF \ Set R = &FF, to use as a mask for drawing each row of STX R \ each character block of the bar, starting with a full \ character's width of 4 pixels CMP T1 \ If A >= T1 then we have passed the threshold where we BCS DL30 \ change bar colour, so jump to DL30 to set A to the \ "high value" colour LDA K+1 \ Set A to K+1, the "low value" colour to use BNE DL31 \ Jump down to DL31 (this BNE is effectively a JMP as A \ will never be zero) .DL30 LDA K \ Set A to K, the "high value" colour to use .DL31 STA COL \ Store the colour of the indicator in COL LDY #2 \ We want to start drawing the indicator on the third \ line in this character row, so set Y to point to that \ row's offset LDX #7 \ Set up a counter in X for the width of the indicator, \ which is 8 characters (each of which is 2 pixels wide, \ to give a total width of 16 pixels) .DL1 LDA Q \ Fetch the indicator value (0-15) from Q into A CMP #2 \ If Q < 2, then we need to draw the end cap of the BCC DL2 \ indicator, which is less than a full character's \ width, so jump down to DL2 to do this SBC #2 \ Otherwise we can draw a 2-pixel wide block, so STA Q \ subtract 2 from Q so it contains the amount of the \ indicator that's left to draw after this character LDA R \ Fetch the shape of the indicator row that we need to \ display from R, so we can use it as a mask when \ painting the indicator. It will be &FF at this point \ (i.e. a full 4-pixel row) .DL5 AND COL \ Fetch the 2-pixel mode 2 colour byte from COL, and \ only keep pixels that have their equivalent bits set \ in the mask byte in A STA (SC),Y \ Draw the shape of the mask on pixel row Y of the \ character block we are processing INY \ Draw the next pixel row, incrementing Y STA (SC),Y INY \ And draw the third pixel row, incrementing Y STA (SC),Y TYA \ Add 6 to Y, so Y is now 8 more than when we started CLC \ this loop iteration, so Y now points to the address ADC #6 \ of the first line of the indicator bar in the next TAY \ character block (as each character is 8 bytes of \ screen memory) DEX \ Decrement the loop counter for the next character \ block along in the indicator BMI DL6 \ If we just drew the last character block then we are \ done drawing, so jump down to DL6 to finish off BPL DL1 \ Loop back to DL1 to draw the next character block of \ the indicator (this BPL is effectively a JMP as A will \ never be negative following the previous BMI) .DL2 EOR #1 \ If we get here then we are drawing the indicator's STA Q \ end cap, so Q is < 2, and this EOR flips the bits, so \ instead of containing the number of indicator columns \ we need to fill in on the left side of the cap's \ character block, Q now contains the number of blank \ columns there should be on the right side of the cap's \ character block LDA R \ Fetch the current mask from R, which will be &FF at \ this point, so we need to turn Q of the columns on the \ right side of the mask to black to get the correct end \ cap shape for the indicator .DL3 ASL A \ Shift the mask left and clear bits 0, 2, 4 and 8, AND #%10101010 \ which has the effect of shifting zeroes from the left \ into each two-bit segment (i.e. xx xx xx xx becomes \ x0 x0 x0 x0, which blanks out the last column in the \ 2-pixel mode 2 character block) DEC Q \ Decrement the counter for the number of columns to \ blank out BPL DL3 \ If we still have columns to blank out in the mask, \ loop back to DL3 until the mask is correct for the \ end cap PHA \ Store the mask byte on the stack while we use the \ accumulator for a bit LDA #0 \ Change the mask so no bits are set, so the characters STA R \ after the one we're about to draw will be all blank LDA #99 \ Set Q to a high number (99, why not) so we will keep STA Q \ drawing blank characters until we reach the end of \ the indicator row PLA \ Restore the mask byte from the stack so we can use it \ to draw the end cap of the indicator JMP DL5 \ Jump back up to DL5 to draw the mask byte on-screen .DL6 INC SC+1 \ Increment the high byte of SC to point to the next INC SC+1 \ character row on-screen (as each row takes up exactly \ two pages of 256 bytes) - so this sets up SC to point \ to the next indicator, i.e. the one below the one we \ just drew .DL9 \ This label is not used but is in the original source RTS \ Return from the subroutine