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Clarify TmpHi, add Listing 3b

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Michaelangel007 2016-01-27 08:29:16 -08:00
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README.md
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@ -1,6 +1,6 @@
#Apple ]\[ //e HGR Font 6502 Assembly Language Tutorial
Revision: 45, Jan 27, 2016.
Revision: 46, Jan 27, 2016.
# Table of Contents
@ -722,6 +722,11 @@ Here's the disassembly of our (hard-coded) DrawChar() program:
090E:A5 E6 LDA HgrHi ; to working pointer
0910:85 F6 STA TmpHi
0912:60 RTS
```
Listing 1:
```assembly
; FUNC: DrawChar()
; PARAM: A = glyph to draw
; PARAM: Y = column to draw at; $0 .. $27 (Columns 0 .. 39) (not modified)
@ -838,6 +843,8 @@ After drawing a character with `DrawChar()` it is handy if we can advance both:
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.
Listing 2a:
```assembly
; FUNC: IncCursorCol1()
; OUTPUT: Y-Register (column) is incremented
@ -865,6 +872,8 @@ One tip for beginner 6502 assembly programmers. It is tempting just to always cl
We change the carry flag state _before_ we do the operation depending on whether we are `adding` or `subtracting`. The `Rule of Thumb` is `CLC before ADD` and `SEC before SUB`. A mnemonic to help you remember is that both `SEC` and `SUB` start with `S`.
Listing 2b:
```assembly
; FUNC: IncCursorCol2()
; OUTPUT: Y-Register (column) is incremented
@ -876,7 +885,7 @@ We change the carry flag state _before_ we do the operation depending on whether
0364:C8 INY
0365:38 SEC ; CLC SBC #1F
0366:A5 F6 LDA TmpHi ; was not obvious that we really
0368:E9 20 SBC #$20 ; meant A - #$20 !!
0368:E9 20 SBC #$20 ; meant: TmpHi = A - #$20 !!
036A:85 F6 STA TmpHi
036C:60 RTS
```
@ -926,12 +935,14 @@ Since we only care about the high byte:
Hmm, we would need to replace `SEC SBC` with `AND OR` which we might think would be a littler faster and takes less code to boot but let's verify our assumption:
Listing 3a:
```assembly
; FUNC: IncCursorCol3()
; OUTPUT: Y-Register (column) is incremented
; Increment the cursor column and move the destination screen pointer back
; up 8 scan lines previously to what it was when DrawChar() was called.
; Version 3
; Version 3a
ORG $0364
0364: IncCursorCol3
0364:C8 INY
@ -960,6 +971,28 @@ vs
```
Nope. Bummer. :-(
We probably should remove that hard-coded HGR page 1.
Listing 3b:
```assembly
; FUNC: IncCursorCol3()
; OUTPUT: Y-Register (column) is incremented
; Increment the cursor column and move the destination screen pointer back
; up 8 scan lines previously to what it was when DrawChar() was called.
; Version 3b
ORG $0364
0364: IncCursorCol3
0364:C8 INY
0365:A5 F6 LDA TmpHi
0367:29 1F AND #%00011111 ; Requires an extra OR
0369:05 E6 ORA HgrHi ; user specified Page 1 or Page 2
036B:85 F6 STA TmpHi
036D:60 RTS
```
At least the timing for `ORA HgrHi` is still 2 clock cycles. :-)
The lessons?
* Verify our assumptions and Profile!
@ -2314,11 +2347,9 @@ Let's write a program to view all the ASCII characters.
```Assembly
.ORG $1050
; <<Forthcoming!>>
```
Here is a table of all the glyphs that we'll eventually fix:
|Glyph|