.vlin12 ; We now draw the middle part of the line (i.e. the part ; between the top and bottom caps) 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 LDY #0 ; We want to start drawing the line from the top pixel ; line in the next character row, so set Y = 0 to use as ; the pixel row number ; Next, we update SC2(1 0) to the address of the next ; row down in the nametable buffer, which we can do by ; adding 32 as there are 32 tiles in each row LDA SC2 ; Set SC2(1 0) = SC2(1 0) + 32 CLC ; ADC #32 ; Starting with the low bytes STA SC2 BCC vlin13 ; And then the high bytes INC SC2+1 .vlin13 ; We jump here from part 2 if the line starts at the top ; of a character block 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 SEC ; Set A = A - 8 SBC #8 ; = R - 8 ; ; So this subtracts 8 pixels (one block) from the number ; of pixels we still have to draw BCS vlin14 ; If the subtraction didn't underflow, then there are at ; least 8 more pixels to draw, so jump to vlin14 to draw ; another block's worth of pixels JMP vlin4 ; The subtraction underflowed, so R is less than 8 and ; we need to stop drawing full-height blocks and draw ; the bottom end of the line, so jump to vlin4 with ; Y = 0 to do just this .vlin14 STA R ; Store the updated number of pixels left to draw, which ; we calculated in the subtraction above LDX #0 ; If the nametable buffer entry is zero for the tile LDA (SC2,X) ; containing the pixels that we want to draw, then a BEQ vlin15 ; pattern has not yet been allocated to this entry, so ; jump to vlin15 to place a pre-rendered pattern into ; the nametable entry CMP #60 ; If A < 60, then the pattern that's already allocated BCC vlin17 ; is either an icon bar pattern, or one of the ; pre-rendered patterns containing horizontal and ; vertical lines, so jump to vlin17 to process drawing ; on top of the pre-rendered pattern ; If we get here then the pattern number already ; allocated to this part of the line is >= 60, which is ; a pattern into which we can draw ; ; 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 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 #0 ; We are going to draw a vertical line from pixel row 0 ; to row 7, so set an index in Y to count up ; We repeat the following code eight times, so it draws ; eight pixels from the top of the character block to ; the bottom 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 INY ; Increment the index in Y 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 INY ; Increment the index in Y 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 INY ; Increment the index in Y 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 INY ; Increment the index in Y 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 INY ; Increment the index in Y 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 INY ; Increment the index in Y 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 INY ; Increment the index in Y 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 JMP vlin12 ; Loop back to move down a row and draw the next block .vlin15 LDA S ; Set A to the pixel column within the character block ; at which we want to draw our line, which we stored in ; S in part 1 CLC ; Patterns 52 to 59 contain pre-rendered patterns as ADC #52 ; follows: ; ; * Pattern 52 has a vertical line in pixel column 0 ; * Pattern 53 has a vertical line in pixel column 1 ; ... ; * Pattern 58 has a vertical line in pixel column 6 ; * Pattern 59 has a vertical line in pixel column 7 ; ; So A contains the pre-rendered pattern number that ; contains an 8-pixel line in pixel column S, and as S ; contains the offset of the pixel column for the line ; we are drawing, this means A contains the correct ; pattern number for this part of the line STA (SC2,X) ; Display the pre-rendered pattern on-screen by setting ; the nametable entry to A .vlin16 JMP vlin12 ; Loop back to move down a row and draw the next block .vlin17 ; If we get here then A <= 59, so the pattern that's ; already allocated is either an icon bar pattern, or ; one of the pre-rendered patterns containing horizontal ; and vertical lines ; ; We jump here with X = 0, so the write to (SC2,X) ; below will work properly STA SC ; Set SC to the number of the pattern that is already ; allocated to this part of the screen, so we can ; retrieve it later LDA firstFreePattern ; If firstFreePattern is zero then we have run out of BEQ vlin16 ; patterns to use for drawing lines and pixels, so jump ; to vlin16 to move down by one character block without ; drawing anything, as we don't have enough patterns to ; draw this part of the line 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 STA (SC2,X) ; Otherwise firstFreePattern contains the number of the ; next available pattern for drawing, so allocate this ; tile to contain the pre-rendered pattern that we want ; to copy by setting the nametable entry to the pattern ; number we just fetched 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 pattern we just fetched ROL SC3+1 STA SC3 LDA SC ; Set A to the number of the pattern that is already ; allocated to this part of the screen, which we stored ; in SC above 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 pre-rendered ROL SC+1 ; pattern that we want to copy STA SC ; We now have a new pattern in SC3(1 0) into which we ; can draw the middle part of our line, so we now need ; to copy the pattern of 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 SC(1 0) to SC3(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 SC(1 0) to SC3(1 0), so ; set a counter in Y 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 .vlin18 LDA (SC),Y ; Draw a pixel at x-coordinate X into the Y-th byte ORA TWOS,X ; of SC(1 0) and store the result in the Y-th byte of STA (SC3),Y ; SC3(1 0), so this copies the pre-rendered pattern, ; superimposes our vertical line on the result and ; stores it in the pattern buffer for the tile we just ; allocated DEY ; Decrement the counter BPL vlin18 ; Loop back until we have copied all eight bytes BMI vlin16 ; Jump to vlin12 via vlin16 to move down a row and draw ; the next blockName: DrawVerticalLine (Part 3 of 3) [Show more] Type: Subroutine Category: Drawing lines Summary: Draw the middle portion of the line from full-height blocks 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 vlin11 in subroutine DrawVerticalLine (Part 2 of 3)
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Label vlin12 is local to this routine
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Label vlin13 is local to this routine
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Label vlin14 is local to this routine
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Label vlin15 is local to this routine
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Label vlin16 is local to this routine
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Label vlin17 is local to this routine
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Label vlin18 is local to this routine
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Label vlin4 in subroutine DrawVerticalLine (Part 2 of 3)