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BBC Micro Elite

Drawing lines: HLOIN

Name: HLOIN [View in context] Type: Subroutine Category: Drawing lines Summary: Draw a horizontal line from (X1, Y1) to (X2, Y1)
We do not draw a pixel at the end point (X2, X1). To understand how this routine works, you might find it helpful to read the deep dive on "Drawing monochrome pixels in mode 4". Returns: Y Y is preserved
.HLOIN STY YSAV \ Store Y into YSAV, so we can preserve it across the \ call to this subroutine LDX X1 \ Set X = X1 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 LDA Y1 \ Set A = Y1 / 8, so A now contains the character row LSR A \ that will contain our horizontal line LSR A LSR A ORA #&60 \ As A < 32, this effectively adds &60 to A, which gives \ us the screen address of the character row (as each \ character row takes up 256 bytes, and the first \ character row is at screen address &6000, or page &60) STA SCH \ Store the page number of the character row in SCH, so \ the high byte of SC is set correctly for drawing our \ line LDA Y1 \ 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 = bits 3-7 of X1 AND #%11111000 TAY .HL1 TXA \ Set T = bits 3-7 of X1, which will contain the AND #%11111000 \ the character number of the start of the line * 8 STA T LDA X2 \ Set A = bits 3-7 of X2, which will contain the AND #%11111000 \ the character number of the end of the line * 8 SEC \ Set A = A - T, which will contain the number of SBC T \ character blocks we need to fill - 1 * 8 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 / 8, so R now contains the number of LSR A \ character blocks we need to fill - 1 LSR A STA R LDA X1 \ Set X = X1 mod 8, which is the horizontal pixel number AND #7 \ within the character block where the line starts (as TAX \ each pixel line in the character block is 8 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 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 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 #%11111111 \ Store a full-width 8-pixel horizontal line in SC(1 0) EOR (SC),Y \ so that it draws the line on-screen, using EOR logic STA (SC),Y \ 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 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 #7 \ of the line, so set X = X2 mod 8, 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 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 LDY YSAV \ Restore Y from YSAV, so that it's preserved across the \ call to this subroutine 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 8, which is the horizontal pixel number AND #7 \ within the character block where the line starts (as TAX \ each pixel line in the character block is 8 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 8, which is the horizontal pixel number AND #7 \ 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 2 to point 5, 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 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 YSAV \ Restore Y from YSAV, so that it's preserved RTS \ Return from the subroutine