.chpr27 ; We jump here from below when the tile we are drawing ; into is not empty 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 character ROL SC+1 ; we are drawing STA SC ; We now copy the pattern data for this character from ; the address in P(2 1) to the pattern buffer address ; in SC(1 0), using OR logic to merge the character with ; the existing contents of the tile LDY #0 ; We want to copy eight bytes of pattern data, as each ; character has eight rows of eight pixels, so set a ; byte index counter in Y ; We repeat the following code eight times, so it copies ; one whole pattern of eight bytes LDA (P+1),Y ; Copy the Y-th pattern byte from P(2 1) to the Y-th ORA (SC),Y ; byte of the pattern buffer in SC(1 0), OR'ing the byte STA (SC),Y ; with the existing contents of the pattern buffer, and INY ; increment the byte counter in Y LDA (P+1),Y ; Copy the Y-th pattern byte from P(2 1) to the Y-th ORA (SC),Y ; byte of the pattern buffer in SC(1 0), OR'ing the byte STA (SC),Y ; with the existing contents of the pattern buffer, and INY ; increment the byte counter in Y LDA (P+1),Y ; Copy the Y-th pattern byte from P(2 1) to the Y-th ORA (SC),Y ; byte of the pattern buffer in SC(1 0), OR'ing the byte STA (SC),Y ; with the existing contents of the pattern buffer, and INY ; increment the byte counter in Y LDA (P+1),Y ; Copy the Y-th pattern byte from P(2 1) to the Y-th ORA (SC),Y ; byte of the pattern buffer in SC(1 0), OR'ing the byte STA (SC),Y ; with the existing contents of the pattern buffer, and INY ; increment the byte counter in Y LDA (P+1),Y ; Copy the Y-th pattern byte from P(2 1) to the Y-th ORA (SC),Y ; byte of the pattern buffer in SC(1 0), OR'ing the byte STA (SC),Y ; with the existing contents of the pattern buffer, and INY ; increment the byte counter in Y LDA (P+1),Y ; Copy the Y-th pattern byte from P(2 1) to the Y-th ORA (SC),Y ; byte of the pattern buffer in SC(1 0), OR'ing the byte STA (SC),Y ; with the existing contents of the pattern buffer, and INY ; increment the byte counter in Y LDA (P+1),Y ; Copy the Y-th pattern byte from P(2 1) to the Y-th ORA (SC),Y ; byte of the pattern buffer in SC(1 0), OR'ing the byte STA (SC),Y ; with the existing contents of the pattern buffer, and INY ; increment the byte counter in Y LDA (P+1),Y ; Copy the Y-th pattern byte from P(2 1) to the Y-th ORA (SC),Y ; byte of the pattern buffer in SC(1 0), OR'ing the byte STA (SC),Y ; with the existing contents of the pattern buffer JMP chpr17 ; Jump to chpr17 to return from the subroutine, as we ; are done printing this character .chpr28 ; If we get here then this is the space view with no ; font loaded, and we have set up P(2 1) to point to the ; pattern data for the character we want to draw LDA #0 ; Set SC+1 = 0 to act as the high byte of SC(1 0) in the STA SC+1 ; calculation below LDA YC ; Set A to the current text cursor row BNE chpr29 ; If the cursor is in row 0, set A = 255 so the value LDA #255 ; of A + 1 is 0 in the calculation below .chpr29 CLC ; Set (SC+1 A) = (A + 1) * 16 ADC #1 ASL A ASL A ASL A ASL A ROL SC+1 SEC ; Set SC(1 0) = (nameBufferHi 0) + (SC+1 A) * 2 + 1 ROL A ; = (nameBufferHi 0) + (A + 1) * 32 + 1 STA SC ; LDA SC+1 ; So SC(1 0) points to the entry in the nametable buffer ROL A ; for the start of the row below the text cursor, plus 1 ADC nameBufferHi STA SC+1 LDY XC ; Set Y to the column of the text cursor, minus one DEY ; So SC(1 0) + Y now points to the nametable entry of ; the tile where we want to draw our character LDA (SC),Y ; If the nametable entry for the tile is not empty, then BNE chpr27 ; jump up to chpr27 to draw our character into the ; existing pattern for this tile LDA firstFreePattern ; If firstFreePattern is zero then we have run out of BEQ chpr30 ; patterns to use for drawing characters, so jump to ; chpr30 to return from the subroutine without printing ; anything STA (SC),Y ; Otherwise firstFreePattern contains the number of the ; next available pattern for drawing, so allocate this ; pattern to cover the character 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 added to ; the nametable the next time we need to draw 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 tile containing the character ROL SC+1 ; we are drawing STA SC ; We now copy the pattern data for this character from ; the address in P(2 1) to the pattern buffer address ; in SC(1 0) LDY #0 ; We want to copy eight bytes of pattern data, as each ; character has eight rows of eight pixels, so set a ; byte index counter in Y ; We repeat the following code eight times, so it copies ; one whole pattern of eight bytes LDA (P+1),Y ; Copy the Y-th pattern byte from P(2 1) to the Y-th STA (SC),Y ; byte of the pattern buffer in SC(1 0), and increment INY ; the byte counter in Y LDA (P+1),Y ; Copy the Y-th pattern byte from P(2 1) to the Y-th STA (SC),Y ; byte of the pattern buffer in SC(1 0), and increment INY ; the byte counter in Y LDA (P+1),Y ; Copy the Y-th pattern byte from P(2 1) to the Y-th STA (SC),Y ; byte of the pattern buffer in SC(1 0), and increment INY ; the byte counter in Y LDA (P+1),Y ; Copy the Y-th pattern byte from P(2 1) to the Y-th STA (SC),Y ; byte of the pattern buffer in SC(1 0), and increment INY ; the byte counter in Y LDA (P+1),Y ; Copy the Y-th pattern byte from P(2 1) to the Y-th STA (SC),Y ; byte of the pattern buffer in SC(1 0), and increment INY ; the byte counter in Y LDA (P+1),Y ; Copy the Y-th pattern byte from P(2 1) to the Y-th STA (SC),Y ; byte of the pattern buffer in SC(1 0), and increment INY ; the byte counter in Y LDA (P+1),Y ; Copy the Y-th pattern byte from P(2 1) to the Y-th STA (SC),Y ; byte of the pattern buffer in SC(1 0), and increment INY ; the byte counter in Y LDA (P+1),Y ; Copy the Y-th pattern byte from P(2 1) to the Y-th STA (SC),Y ; byte of the pattern buffer in SC(1 0) .chpr30 JMP chpr17 ; Jump to chpr17 to return from the subroutine, as we ; are done printing this character .chpr31 ; If we get here then this is the space view and the ; text cursor is on row 0 LDA #33 ; Set SC(1 0) to the address of tile 33 in the nametable STA SC ; buffer, which is the first tile on row 1 LDA nameBufferHi STA SC+1 LDY XC ; Set Y to the column of the text cursor - 1 DEY JMP chpr15 ; Jump up to chpr15 to continue drawing the characterName: CHPR (Part 6 of 6) [Show more] Type: Subroutine Category: Text Summary: Print a character in the space view when the relevant font is not loaded, merging the text with whatever is already on-screen Deep dive: Fonts in NES EliteContext: See this subroutine in context in the source code References: No direct references to this subroutine in this source file
[X]
Label chpr15 in subroutine CHPR (Part 3 of 6)
[X]
Label chpr17 in subroutine CHPR (Part 3 of 6)
[X]
Label chpr27 is local to this routine
[X]
Label chpr29 is local to this routine
[X]
Label chpr30 is local to this routine
[X]
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)
[X]
Variable nameBufferHi in workspace ZP
High byte of the address of the current nametable buffer ($70 or $74)
[X]
Variable pattBufferHiDiv8 in workspace ZP
High byte of the address of the current pattern buffer ($60 or $68) divided by 8