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Elite on the BBC Micro

Drawing lines: HLOIN (6502SP version)

Name: HLOIN [View in context] Type: Subroutine [Compare versions] Category: Drawing lines Summary: Implement the OSWORD 247 command (draw the sun lines in the horizontal line buffer in orange)
This routine is run when the parasite sends an OSWORD 247 command with parameters in the block at OSSC(1 0). It draws a horizontal orange line (or a collection of lines) in the space view. The parameters match those put into the HBUF block in the parasite. Each line is drawn from (X1, Y1) to (X2, Y1), and lines are drawn in orange. We do not draw a pixel at the end point (X2, X1). Arguments: OSSC(1 0) A parameter block as follows: * Byte #0 = The size of the parameter block being sent * Byte #2 = The x-coordinate of the first line's starting point * Byte #3 = The x-coordinate of the first line's end point * Byte #4 = The y-coordinate of the first line * Byte #5 = The x-coordinate of the second line's starting point * Byte #6 = The x-coordinate of the second line's end point * Byte #7 = The y-coordinate of the second line and so on Other entry points: HLOIN3 Draw a line from (X1, Y1) to (X2, Y1) in the current colour (we need to set Q = Y2 + 1 before calling HLOIN3 so only one line is drawn)
.HLOIN LDY #0 \ Fetch byte #0 from the parameter block (which gives LDA (OSSC),Y \ size of the parameter block) and store it in Q STA Q INY \ Increment Y to point to byte #2, which is where the INY \ line coordinates start .HLLO LDA (OSSC),Y \ Fetch the Y-th byte from the parameter block (the STA X1 \ line's X1 coordinate) and store it in X1 and X TAX INY \ Fetch the Y+1-th byte from the parameter block (the LDA (OSSC),Y \ line's X2 coordinate) and store it in X2 STA X2 INY \ Fetch the Y+2-th byte from the parameter block (the LDA (OSSC),Y \ line's Y1 coordinate) and store it in Y1 STA Y1 STY Y2 \ Store the parameter block offset for this line's Y1 \ coordinate in Y2, so we know where to fetch the next \ line from in the parameter block once we have drawn \ this one AND #3 \ Set A to the correct order of red/yellow pixels to TAY \ make this line an orange colour (by using bits 0-1 of LDA orange,Y \ the pixel y-coordinate as the index into the orange \ lookup table) .HLOIN3 STA S \ Store the line colour in S CPX X2 \ If X1 = X2 then the start and end points are the same, BEQ HL6 \ so return from the subroutine (as HL6 contains an RTS) BCC HL5 \ If X1 < X2, jump to HL5 to skip the following code, as \ (X1, Y1) is already the left point LDA X2 \ Swap the values of X1 and X2, so we know that (X1, Y1) STA X1 \ is on the left and (X2, Y1) is on the right STX X2 TAX \ Set X = X1 .HL5 DEC X2 \ Decrement X2 so we do not draw a pixel at the end \ point LDY Y1 \ Look up the page number of the character row that LDA ylookup,Y \ contains the pixel with the y-coordinate in Y1, and STA SC+1 \ store it in SC+1, so the high byte of SC is set \ correctly for drawing our line TYA \ Set A = Y1 mod 8, which is the pixel row within the AND #7 \ character block at which we want to draw our line (as \ each character block has 8 rows) STA SC \ Store this value in SC, so SC(1 0) now contains the \ screen address of the far left end (x-coordinate = 0) \ of the horizontal pixel row that we want to draw our \ horizontal line on TXA \ Set Y = 2 * bits 2-6 of X1 AND #%11111100 \ ASL A \ and shift bit 7 of X1 into the C flag TAY BCC P%+4 \ If bit 7 of X1 was set, so X1 > 127, increment the INC SC+1 \ high byte of SC(1 0) to point to the second page on \ this screen row, as this page contains the right half \ of the row .HL1 TXA \ Set T = bits 2-7 of X1, which will contain the AND #%11111100 \ the character number of the start of the line * 4 STA T LDA X2 \ Set A = bits 2-7 of X2, which will contain the AND #%11111100 \ the character number of the end of the line * 4 SEC \ Set A = A - T, which will contain the number of SBC T \ character blocks we need to fill - 1 * 4 BEQ HL2 \ If A = 0 then the start and end character blocks are \ the same, so the whole line fits within one block, so \ jump down to HL2 to draw the line \ Otherwise the line spans multiple characters, so we \ start with the left character, then do any characters \ in the middle, and finish with the right character LSR A \ Set R = A / 4, so R now contains the number of LSR A \ character blocks we need to fill - 1 STA R LDA X1 \ Set X = X1 mod 4, which is the horizontal pixel number AND #3 \ within the character block where the line starts (as TAX \ each pixel line in the character block is 4 pixels \ wide) LDA TWFR,X \ Fetch a ready-made byte with X pixels filled in at the \ right end of the byte (so the filled pixels start at \ point X and go all the way to the end of the byte), \ which is the shape we want for the left end of the \ line AND S \ Apply the pixel mask in A to the four-pixel block of \ coloured pixels in S, so we now know which bits to set \ in screen memory to paint the relevant pixels in the \ required colour EOR (SC),Y \ Store this into screen memory at SC(1 0), using EOR STA (SC),Y \ logic so it merges with whatever is already on-screen, \ so we have now drawn the line's left cap TYA \ Set Y = Y + 8 so (SC),Y points to the next character ADC #8 \ block along, on the same pixel row as before TAY BCS HL7 \ If the above addition overflowed, then we have just \ crossed over from the left half of the screen into the \ right half, so call HL7 to increment the high byte in \ SC+1 so that SC(1 0) points to the page in screen \ memory for the right half of the screen row. HL7 also \ clears the C flag and jumps back to HL8, so this acts \ like a conditional JSR instruction .HL8 LDX R \ Fetch the number of character blocks we need to fill \ from R DEX \ Decrement the number of character blocks in X BEQ HL3 \ If X = 0 then we only have the last block to do (i.e. \ the right cap), so jump down to HL3 to draw it CLC \ Otherwise clear the C flag so we can do some additions \ while we draw the character blocks with full-width \ lines in them .HLL1 LDA S \ Store a full-width 4-pixel horizontal line of colour S EOR (SC),Y \ in SC(1 0) so that it draws the line on-screen, using STA (SC),Y \ EOR logic so it merges with whatever is already \ on-screen TYA \ Set Y = Y + 8 so (SC),Y points to the next character ADC #8 \ block along, on the same pixel row as before TAY BCS HL9 \ If the above addition overflowed, then we have just \ crossed over from the left half of the screen into the \ right half, so call HL9 to increment the high byte in \ SC+1 so that SC(1 0) points to the page in screen \ memory for the right half of the screen row. HL9 also \ clears the C flag and jumps back to HL10, so this acts \ like a conditional JSR instruction .HL10 DEX \ Decrement the number of character blocks in X BNE HLL1 \ Loop back to draw more full-width lines, if we have \ any more to draw .HL3 LDA X2 \ Now to draw the last character block at the right end AND #3 \ of the line, so set X = X2 mod 3, which is the TAX \ horizontal pixel number where the line ends LDA TWFL,X \ Fetch a ready-made byte with X pixels filled in at the \ left end of the byte (so the filled pixels start at \ the left edge and go up to point X), which is the \ shape we want for the right end of the line AND S \ Apply the pixel mask in A to the four-pixel block of \ coloured pixels in S, so we now know which bits to set \ in screen memory to paint the relevant pixels in the \ required colour EOR (SC),Y \ Store this into screen memory at SC(1 0), using EOR STA (SC),Y \ logic so it merges with whatever is already on-screen, \ so we have now drawn the line's right cap .HL6 LDY Y2 \ Set Y to the parameter block offset for this line's Y1 \ coordinate, which we stored in Y2 before we drew the \ line INY \ Increment Y so that it points to the first parameter \ for the next line in the parameter block CPY Q \ If Y = Q then we have drawn all the lines in the BEQ P%+5 \ parameter block, so skip the next instruction to \ return from the subroutine JMP HLLO \ There is another line in the parameter block after the \ one we just drew, so jump to HLLO with Y pointing to \ the new line's coordinates, so we can draw it RTS \ Return from the subroutine .HL2 \ If we get here then the entire horizontal line fits \ into one character block LDA X1 \ Set X = X1 mod 4, which is the horizontal pixel number AND #3 \ within the character block where the line starts (as TAX \ each pixel line in the character block is 4 pixels \ wide) LDA TWFR,X \ Fetch a ready-made byte with X pixels filled in at the STA T \ right end of the byte (so the filled pixels start at \ point X and go all the way to the end of the byte) LDA X2 \ Set X = X2 mod 4, which is the horizontal pixel number AND #3 \ where the line ends TAX LDA TWFL,X \ Fetch a ready-made byte with X pixels filled in at the \ left end of the byte (so the filled pixels start at \ the left edge and go up to point X) AND T \ We now have two bytes, one (T) containing pixels from \ the starting point X1 onwards, and the other (A) \ containing pixels up to the end point at X2, so we can \ get the actual line we want to draw by AND'ing them \ together. For example, if we want to draw a line from \ point 1 to point 2 (within the row of 4 pixels \ numbered from 0 to 3), we would have this: \ \ T = %00111111 \ A = %11111100 \ T AND A = %00111100 \ \ so if we stick T AND A in screen memory, that's what \ we do here, setting A = A AND T AND S \ Apply the pixel mask in A to the four-pixel block of \ coloured pixels in S, so we now know which bits to set \ in screen memory to paint the relevant pixels in the \ required colour EOR (SC),Y \ Store our horizontal line byte into screen memory at STA (SC),Y \ SC(1 0), using EOR logic so it merges with whatever is \ already on-screen LDY Y2 \ Set Y to the parameter block offset for this line's Y1 \ coordinate, which we stored in Y2 before we drew the \ line INY \ Increment Y so that it points to the first parameter \ for the next line in the parameter block CPY Q \ If Y = Q then we have drawn all the lines in the BEQ P%+5 \ parameter block, so skip the next instruction to \ return from the subroutine JMP HLLO \ There is another line in the parameter block after the \ one we just drew, so jump to HLLO with Y pointing to \ the new line's coordinates, so we can draw it RTS \ Return from the subroutine .HL7 INC SC+1 \ We have just crossed over from the left half of the \ screen into the right half, so increment the high byte \ in SC+1 so that SC(1 0) points to the page in screen \ memory for the right half of the screen row CLC \ Clear the C flag (as HL7 is called with the C flag \ set, which this instruction reverts) JMP HL8 \ Jump back to HL8, just after the instruction that \ called HL7 .HL9 INC SC+1 \ We have just crossed over from the left half of the \ screen into the right half, so increment the high byte \ in SC+1 so that SC(1 0) points to the page in screen \ memory for the right half of the screen row CLC \ Clear the C flag (as HL9 is called with the C flag \ set, which this instruction reverts) JMP HL10 \ Jump back to HL10, just after the instruction that \ called HL9