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WIP: cleanup asm indent
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michaelangel007
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README.md
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README.md
@ -614,42 +614,41 @@ Before we can start a simple `DrawChar(char c)` function, we also first need to
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HgrHi EQU $E6
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TmpLo EQU $F5
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TmpHi EQU $F6
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Font EQU $6000
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```
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Here's the disassembly of our (hard-coded) DrawChar() program:
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```assembly
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; FUNC: DrawChar()
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; NOTES: A, X, Y is destroyed
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ORG $0300
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300:20 66 03 DrawChar JSR HgrToTmpPtr
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303:A9 00 LDA #00 ; glyph 'c' to draw (not used yet)
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305:A0 00 LDY #00 ; Y = column to draw at (hard-coded)
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307:4C 52 03 JMP _DrawChar
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0300: DrawChar
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0300:20 66 03 JSR HgrToTmpPtr
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0303:A9 00 LDA #00 ; glyph 'c' to draw (not used yet)
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0305:A0 00 LDY #00 ; Y = column to draw at (hard-coded)
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0307:4C 52 03 JMP _DrawChar
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ORG $0352
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352: _DrawChar
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352:A2 00 LDX #0 ; next instruction is Self-Modifying!
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354:BD 00 00 LoadFont LDA $0000,X ; A = font[ offset + i ]
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357:91 F5 STA (TmpLo),Y ; screen[col] = A
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359:18 CLC
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35A:A5 F6 LDA TmpHi
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35C:69 04 ADC #4 ; screen += 0x400
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35E:85 F6 STA TmpHi
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360:E8 INX
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361:E0 08 CPX #8
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363:D0 EF BNE LoadFont
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365:60 RTS
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0352: _DrawChar
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0352:A2 00 LDX #0 ; next instruction is Self-Modifying!
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0354: _LoadFont ; A = font[ offset ]
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0354:BD 00 62 LDA Font+#$200$,X
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0357:91 F5 STA (TmpLo),Y ; screen[col] = A
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0359:18 CLC
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035A:A5 F6 LDA TmpHi
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035C:69 04 ADC #4 ; screen += 0x400
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035E:85 F6 STA TmpHi
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0360:E8 INX
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0361:E0 08 CPX #8
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0363:D0 EF BNE _LoadFont
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0365:60 RTS
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; FUNC: HgrToTmpPtr()
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ORG $0366
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366: HgrToTmpPtr
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366:A5 E5 LDA HgrLo ; Copy initial screen
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368:85 F5 STA TmpLo ; destination pointer
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36A:A5 E6 LDA HgrHi ; to working pointer
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36C:85 F6 STA TmpHi
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36D:60 RTS
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0366: HgrToTmpPtr
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0366:A5 E5 LDA HgrLo ; Copy initial screen
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0368:85 F5 STA TmpLo ; destination pointer
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036A:A5 E6 LDA HgrHi ; to working pointer
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036C:85 F6 STA TmpHi
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036D:60 RTS
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```
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Enter in:
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@ -687,7 +686,9 @@ Enter in:
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This works because we are using the 6502 Indirect Zero-Page Y addressing mode to store the destination pixels with the `STA` instruction. Since the Y-register must _always_ be used in this addressing mode we get a column offset "for free." :-)
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357: STA ($F5),Y ; screen[col] = A
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```assembly
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0357:91 F5 STA (TmpLo),Y ; screen[col] = A
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```
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Here's the C pseudo-code of the assembly code:
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@ -705,31 +706,38 @@ Here's the C pseudo-code of the assembly code:
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Since the Y-register controls the column we can inline this function and have the caller take care of setting the Y-Register before calling DrawChar().
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```assmebly
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LDY #column
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```
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After drawing a character with `DrawChar()` it is handy if we can advance both:
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* the column of the cursor
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* the pointer to the screen where the next glyph will be drawn
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Notice how after 8 scan lines we end up with and `Tmp` address of $4xxx (or $6xxx if we were drawing to HGR page 2.) This means we need to subtract off $20 from the top byte of the 16-bit address to the temp destination screen pointer.
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```assembly
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; FUNC: IncCursorCol()
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; OUTPUT: Y-Register (column) is incremented
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; Increment the cursor column and move the destination screen pointer back
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; up 8 scan lines previously to what it was when DrawChar() was called.
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; Version 1
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ORG $0370
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370:C8 IncCursorCol INY
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371:18 CLC
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372:A5 F6 LDA $F6
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374:E9 1F SBC #$1F
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376:85 F6 STA $F6
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378:60 RTS
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0370: IncCursorCol1
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0370:C8 INY
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0371:18 CLC
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0372:A5 F6 LDA TmpHi
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0374:E9 1F SBC #$1F
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0376:85 F6 STA TmpHi
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0378:60 RTS
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```
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Enter in:
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370:C8 18 A5 F6 E9 1F 85 F6 60
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## DrawChar() version 2
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The glyph to draw is currently hard-coded to $40 (`@`). The pointer to the start of this glyph is located at:
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@ -972,8 +980,10 @@ We now have an ASCII char inspector!
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Let's fix it up to print the hex value of the current character we are inspecting:
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```assembly
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1010:20 37 10 JSR $1037
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ORG $1010
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1010:20 37 10 JSR Patch
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ORG $1037
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1037: Patch
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1037:48 PHA ; save c
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1038:20 3B 03 JSR DrawCharCol
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103B:68 PLA ; restore c so we can print it in hex
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@ -997,7 +1007,7 @@ Let's fix it up to print the hex value of the current character we are inspectin
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104A:AA TAX ;
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104B:20 66 03 JSR HgrToTmpPtr
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104E:BD 58 10 LDA NIB2HEX,X ; nibble to ASCII
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1051:C8 INY ; IncCursorCol()
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1051:C8 INY ; partial IncCursorCol()
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1052:20 3B 03 JSR DrawCharCol
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1055:60 RTS
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ORG $0358
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@ -1044,6 +1054,8 @@ Let's use IncCursorCol() to automatically advance the cusor. We'll also add a s
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And the new code to draw a space before the hex num:
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```assembly
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ORG $1037
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1037: Patch
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1037:48 PHA ; save c
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1038:20 10 03 JSR PrintChar ;
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103B:A9 20 LDA ' ' ; Draw whitespace
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@ -1075,10 +1087,19 @@ And the new code to draw a space before the hex num:
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1064:43 44 45 46
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```
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Here's the full updated version.
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Enter in:
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310:20 3B 03 4C 70 03
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1037:48
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1000:A9 00 85 FE A9 00 85 F5
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1008:A9 20 85 F6 A5 FE A0 00
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1010:20 37 10 AD 00 C0 10 FB
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1018:8D 10 C0 C9 88 D0 0A C6
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1020:FE A5 FE 29 7F 85 FE 10
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1028:DB C9 95 D0 05 E6 FE 18
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1030:90 EF C9 9B D0 DD 60 48
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1038:20 10 03 A9 20 20 10 03
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1040:68 48 6A 6A 6A 6A 20 4D
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1048:10 68 4C 4D 10 29 0F AA
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