.vlin2 ; If we get here then we need to move down by one ; character block without drawing anything, and then ; move on to drawing the middle portion of the line STY T ; Set A = R + Y LDA R ; = pixels to left draw + current pixel row ADC T ; ; So A contains the total number of pixels left to draw ; in our line SBC #7 ; At this point the C flag is clear, as the above ; addition won't overflow, so this sets A = R + Y - 8 ; and sets the flags accordingly BCC vlin3 ; If the above subtraction didn't underflow then ; R + Y < 8, so there is less than one block height to ; draw, so there would be nothing more to draw after ; moving down one line, so jump to vlin3 to return ; from the subroutine JMP vlin12 ; Jump to vlin12 to move on drawing the middle ; portion of the line .vlin3 LDY YSAV ; Restore Y from YSAV, so that it's preserved RTS ; Return from the subroutine .vlin4 ; We now draw either the top end or the bottom end of ; the line into the nametable entry in SC2(1 0) STY Q ; Set Q to the pixel row of the top of the line that we ; want to draw (which will be Y1 mod 8 for the top end ; of the line, or 0 for the bottom end of the line) ; ; For the top end of the line, we draw down from row ; Y1 mod 8 to the bottom of the character block, which ; will correctly draw the top end of the line ; ; For the bottom end of the line, we draw down from row ; 0 until R runs down, which will correctly draw the ; bottom end of the line SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure ; the PPU to use nametable 0 and pattern table 0 LDX #0 ; If the nametable buffer entry is non-zero for the tile LDA (SC2,X) ; containing the pixels that we want to draw, then a BNE vlin6 ; pattern has already been allocated to this entry, so ; skip the following LDA firstFreePattern ; If firstFreePattern is zero then we have run out of BEQ vlin5 ; patterns to use for drawing lines and pixels, so jump ; to vlin2 via vlin5 to move on to the next character ; block down, as we don't have enough patterns to draw ; the top block of the line STA (SC2,X) ; Otherwise firstFreePattern contains the number of the ; next available pattern for drawing, so allocate this ; pattern to cover the pixels that we want to draw by ; setting the nametable entry to the pattern number we ; just fetched INC firstFreePattern ; Increment firstFreePattern to point to the next ; available pattern for drawing, so it can be used the ; next time we need to draw lines or pixels into a ; pattern JMP vlin8 ; Jump to vlin8 to draw the line, starting by drawing ; the top end into the newly allocated pattern number ; in A .vlin5 JMP vlin2 ; Jump to vlin2 to move on to the next character block ; down .vlin6 CMP #60 ; If A >= 60, then the pattern that's already allocated BCS vlin8 ; is one of the patterns we have reserved for drawing, ; so jump to vlin8 to draw the line, starting by drawing ; the top end into the pattern number in A CMP #37 ; If A < 37, then the pattern that's already allocated BCC vlin5 ; is one of the icon bar pattern, so jump to vlin2 via ; vlin5 to move down by one character block without ; drawing anything, as we can't draw on the icon bar ; If we get here then 37 <= A <= 59, so the pattern ; that's already allocated is one of the pre-rendered ; patterns containing horizontal and vertical lines ; ; We don't want to draw over the top of the pre-rendered ; patterns as that will break them, so instead we make a ; copy of the pre-rendered pattern into a newly ; allocated pattern, and then draw our line into the ; this new pattern, thus preserving what's already shown ; on-screen while still drawing our new line LDX pattBufferHiDiv8 ; Set SC3(1 0) = (pattBufferHiDiv8 A) * 8 STX SC3+1 ; = (pattBufferHi A) + A * 8 ASL A ; ROL SC3+1 ; So SC3(1 0) is the address in the pattern buffer for ASL A ; pattern number A (as each pattern contains 8 bytes of ROL SC3+1 ; pattern data), which means SC3(1 0) points to the ASL A ; pattern data for the tile containing the pre-rendered ROL SC3+1 ; pattern that we want to copy STA SC3 LDA firstFreePattern ; If firstFreePattern is zero then we have run out of BEQ vlin5 ; patterns for drawing lines and pixels, so jump to ; vlin2 via vlin5 to move down by one character block ; without drawing anything, as we don't have enough ; patterns to draw the top end of the line LDX #0 ; Otherwise firstFreePattern contains the number of the STA (SC2,X) ; next available pattern for drawing, so allocate this ; pattern to cover the pixels that we want to copy by ; setting the nametable entry to the pattern number we ; just fetched INC firstFreePattern ; Increment firstFreePattern to point to the next ; available pattern for drawing, so it can be used the ; next time we need to draw lines or pixels into a ; pattern LDX pattBufferHiDiv8 ; Set SC(1 0) = (pattBufferHiDiv8 A) * 8 STX SC+1 ; = (pattBufferHi 0) + A * 8 ASL A ; ROL SC+1 ; So SC(1 0) is the address in the pattern buffer for ASL A ; pattern number A (as each pattern contains 8 bytes of ROL SC+1 ; pattern data), which means SC(1 0) points to the ASL A ; pattern data for the pattern we just fetched ROL SC+1 STA SC ; We now have a new pattern in SC(1 0) into which we can ; draw the top end of our line, so we now need to copy ; the pre-rendered pattern that we want to draw on top ; of ; ; Each pattern is made up of eight bytes, so we simply ; need to copy eight bytes from SC3(1 0) to SC(1 0) STY T ; Store Y in T so we can retrieve it after the following ; loop LDY #7 ; We now copy eight bytes from SC3(1 0) to SC(1 0), so ; set a counter in Y .vlin7 LDA (SC3),Y ; Copy the Y-th byte of SC3(1 0) to the Y-th byte of STA (SC),Y ; SC(1 0) DEY ; Decrement the counter BPL vlin7 ; Loop back until we have copied all eight bytes LDY T ; Restore the value of Y from before the loop, so it ; once again contains the pixel row offset within the ; each character block for the line we are drawing JMP vlin9 ; Jump to hlin8 to draw the top end of the line into ; the tile that we just copied .vlin8 ; If we get here then we have either allocated a new ; pattern number for the line, or the pattern number ; already allocated to this part of the line is >= 60, ; which is a pattern into which we can draw ; ; In either case the pattern number is in A LDX pattBufferHiDiv8 ; Set SC(1 0) = (pattBufferHiDiv8 A) * 8 STX SC+1 ; = (pattBufferHi 0) + A * 8 ASL A ; ROL SC+1 ; So SC(1 0) is the address in the pattern buffer for ASL A ; pattern number A (as each pattern contains 8 bytes of ROL SC+1 ; pattern data), which means SC(1 0) points to the ASL A ; pattern data for the tile containing the line we are ROL SC+1 ; drawing STA SC .vlin9 LDX S ; Set X to the pixel column within the character block ; at which we want to draw our line, which we stored in ; S in part 1 LDY Q ; Set Y to y-coordinate of the start of the line, which ; we stored in Q above LDA R ; If the height remaining in R is 0 then we have no more BEQ vlin11 ; line to draw, so jump to vlin11 to return from the ; subroutine .vlin10 LDA (SC),Y ; Draw a pixel at x-coordinate X into the Y-th byte ORA TWOS,X ; of SC(1 0) STA (SC),Y DEC R ; Decrement the height remaining counter in R, as we ; just drew a pixel BEQ vlin11 ; If the height remaining in R is 0 then we have no more ; line to draw, so jump to vlin11 to return from the ; subroutine INY ; Increment the y-coordinate in Y so we move down the ; line by one pixel CPY #8 ; If Y < 8, loop back to vlin10 draw the next pixel as BCC vlin10 ; we haven't yet reached the bottom of the character ; block containing the line's top end BCS vlin12 ; If Y >= 8 then we have drawn our vertical line from ; the starting point to the bottom of the character ; block containing the line's top end, so jump to vlin12 ; to move down one row to draw the middle portion of the ; line .vlin11 LDY YSAV ; Restore Y from YSAV, so that it's preserved RTS ; Return from the subroutineName: DrawVerticalLine (Part 2 of 3) [Show more] Type: Subroutine Category: Drawing lines Summary: Draw the top end or bottom end of the line Deep dive: Drawing lines in the NES versionContext: See this subroutine in context in the source code References: No direct references to this subroutine in this source file
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Macro SETUP_PPU_FOR_ICON_BAR (category: PPU)
If the PPU has started drawing the icon bar, configure the PPU to use nametable 0 and pattern table 0
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Variable TWOS (category: Drawing pixels)
Ready-made single-pixel character row bytes for the space view
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Variable firstFreePattern in workspace ZP
Contains the number of the first free pattern in the pattern buffer that we can draw into next (or 0 if there are no free patterns)
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Variable pattBufferHiDiv8 in workspace ZP
High byte of the address of the current pattern buffer ($60 or $68) divided by 8
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Label vlin10 is local to this routine
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Label vlin11 is local to this routine
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Label vlin12 in subroutine DrawVerticalLine (Part 3 of 3)
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Label vlin2 is local to this routine
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Label vlin3 is local to this routine
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Label vlin5 is local to this routine
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Label vlin6 is local to this routine
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Label vlin7 is local to this routine
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Label vlin8 is local to this routine
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Label vlin9 is local to this routine