; We now draw the middle part of the line (i.e. the part ; between the left and right caps) .hlin11 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 zero for the tile LDA (SC2,X) ; containing the pixels that we want to draw, then a BEQ hlin13 ; pattern has not yet been allocated to this entry, so ; jump to hlin13 to allocate a new pattern ; If we get here then A contains the pattern number ; that's already allocated to this part of the line in ; the nametable buffer CMP #60 ; If A < 60, then the pattern that's already allocated BCC hlin15 ; is either an icon bar pattern, or one of the ; pre-rendered patterns containing horizontal and ; vertical lines, so jump to hlin15 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 LDA #%11111111 ; Set A to a pixel byte containing eight pixels in a row EOR (SC),Y ; Store this into the pattern buffer at SC(1 0), using STA (SC),Y ; EOR logic so it merges with whatever is already ; on-screen, so we have now drawn one character block ; of the middle portion of the line .hlin12 INC SC2 ; Increment SC2(1 0) to point to the next tile number to BNE P%+4 ; the right in the nametable buffer INC SC2+1 DEC R ; Decrement the number of character blocks in which we ; need to draw, as we have just drawn one block BNE hlin11 ; If there are still more character blocks to draw, loop ; back to hlin11 to draw the next one JMP hlin17 ; Otherwise we have finished drawing the middle portion ; of the line, so jump to hlin17 to draw the right end ; of the line .hlin13 ; If we get here then there is no pattern allocated to ; the part of the line we want to draw, so we can use ; one of the pre-rendered patterns that contains an ; 8-pixel horizontal line on the correct pixel row ; ; We jump here with X = 0 TYA ; Set A = Y + 37 CLC ; ADC #37 ; Patterns 37 to 44 contain pre-rendered patterns as ; follows: ; ; * Pattern 37 has a horizontal line on pixel row 0 ; * Pattern 38 has a horizontal line on pixel row 1 ; ... ; * Pattern 43 has a horizontal line on pixel row 6 ; * Pattern 44 has a horizontal line on pixel row 7 ; ; So A contains the pre-rendered pattern number that ; contains an 8-pixel line on pixel row Y, and as Y ; contains the offset of the pixel row 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 JMP hlin12 ; Jump up to hlin12 to move on to the next character ; block to the right .hlin14 ; If we get here then A + Y = 50, which means we can ; alter the current pre-rendered pattern to draw our ; line ; ; This is how it works. Patterns 44 to 51 contain ; pre-rendered patterns as follows: ; ; * Pattern 44 has a horizontal line on pixel row 7 ; * Pattern 45 is filled from pixel row 7 to row 6 ; * Pattern 46 is filled from pixel row 7 to row 5 ; ... ; * Pattern 50 is filled from pixel row 7 to row 1 ; * Pattern 51 is filled from pixel row 7 to row 0 ; ; Y contains the number of the pixel row for the line we ; are drawing, so if A + Y = 50, this means: ; ; * We want to draw pixel row 0 on top of pattern 50 ; * We want to draw pixel row 1 on top of pattern 49 ; ... ; * We want to draw pixel row 5 on top of pattern 45 ; * We want to draw pixel row 6 on top of pattern 44 ; ; In other words, if A + Y = 50, then we want to draw ; the pixel row just above the rows that are already ; filled in the pre-rendered pattern, which means we ; can simply swap the pre-rendered pattern to the next ; one in the list (e.g. going from four filled lines to ; five filled lines, for example) ; ; We jump here with a BEQ, so the C flag is set for the ; following addition, so the C flag can be used as the ; plus 1 in the two's complement calculation TYA ; Set A = 51 + C + ~Y EOR #$FF ; = 51 + (1 + ~Y) ADC #51 ; = 51 - Y ; ; So A contains the number of the pre-rendered pattern ; that has our horizontal line drawn on pixel row Y, and ; all the lines below that filled, which is what we want STA (SC2,X) ; Display the pre-rendered pattern on-screen by setting ; the nametable entry to A INC SC2 ; Increment SC2(1 0) to point to the next tile number to BNE P%+4 ; the right in the nametable buffer INC SC2+1 DEC R ; Decrement the number of character blocks in which we ; need to draw, as we have just drawn one block BNE hlin11 ; If there are still more character blocks to draw, loop ; back to hlin11 to draw the next one JMP hlin17 ; Otherwise we have finished drawing the middle portion ; of the line, so jump to hlin17 to draw the right end ; of the line .hlin15 ; 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 the C flag clear, so the addition ; below will work correctly, and with X = 0, so the ; write to (SC2,X) will also work properly STA SC ; Set SC to the number of the tile that is already ; allocated to this part of the screen, so we can ; retrieve it later TYA ; If A + Y = 50, then we are drawing our line just ADC SC ; above the top line of a pre-rendered pattern that is CMP #50 ; filled from the bottom row to the row just below Y, BEQ hlin14 ; so jump to hlin14 to switch this tile to another ; pre-rendered pattern that contains the line we want to ; draw (see hlin14 for a full explanation of this logic) ; 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, but ; isn't one that we can simply replace with another ; pre-rendered pattern ; ; 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 LDA firstFreePattern ; If firstFreePattern is zero then we have run out of BEQ hlin12 ; patterns to use for drawing lines and pixels, so jump ; to hlin12 to move right 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 left 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 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 .hlin16 LDA (SC),Y ; Copy the Y-th byte of SC(1 0) to the Y-th byte of STA (SC3),Y ; SC3(1 0) DEY ; Decrement the counter BPL hlin16 ; 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 LDA #%11111111 ; Set A to a pixel byte containing eight pixels in a row EOR (SC3),Y ; Store this into the pattern buffer at SC3(1 0), using STA (SC3),Y ; EOR logic so it merges with whatever is already ; on-screen, so we have now drawn one character block ; of the middle portion of the line JMP hlin12 ; Loop back to hlin12 to continue drawing the line in ; the next character block to the rightName: HLOIN (Part 3 of 5) [Show more] Type: Subroutine Category: Drawing lines Summary: Draw the middle part 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 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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Label hlin11 is local to this routine
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Label hlin12 is local to this routine
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Label hlin13 is local to this routine
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Label hlin14 is local to this routine
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Label hlin15 is local to this routine
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Label hlin16 is local to this routine
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Label hlin17 in subroutine HLOIN (Part 4 of 5)
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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