Updated IncreaseScorePackedBCD, closes #127 (#128)

* Simplifying IncreaseScorePackedBCD as per #127

* Updated and extended prose to match code adjustments for #127

Also changed a link because that old link was too much text when gbz80(7) explains the same thing in such a beautifully concise way. This allowed explaining the use of ADD 1 instead of INC.

* Improved wording to be clearer

* Improved wording to be even clearer

Author: nummacway

src/part2/bcd.md

@@ -38,17 +38,21 @@ Don't worry about the call to `UpdateScoreBoard`, we'll get into that in a bit.
 ```
 
 Let's have a look at what's going on there:
-We set A to 1 and clear the carry flag
-We add the score variable (contents of memory location `wScore`) to a, so now A has our increased score.
+
+First we store the score address (`wScore`) in HL. This allows us to use instructions like `LD A, [HL]` and `LD [HL], A`. In total this is 1 M-cycle and 1 byte more efficient than if we provided the address itself when loading and storing the score (`LD A, [wScore]` and `LD [wScore], A` respectively).
+
+Then we load the the score from that address into register A, the only register to be affected by `ADD`. This means that we can now increment it by 1 using exactly that instruction.
 
 So far so good, but what if the score was 9 and we add 1? The processor thinks in binary only and will do the following math:
 
 `%00001001` + `%00000001` = `%00001010` = `$A`
 
 That's a hexadecimal representation of 10, and we need to adjust it to become decimal. `DAA` or "Decimal Adjust after Addition," does just that.
-After executing `DAA` our accumulator will be adjusted from `%00001010` to `%00010000`; a 1 in the left nibble and a 0 in the right one. A more detailed article about `DAA` on the Game Boy can be found [here](https://blog.ollien.com/posts/gb-daa/).
+After executing `DAA` our accumulator will be adjusted from `%00001010` to `%00010000`; a 1 in the left nibble and a 0 in the right one. `DAA`'s exact behaviour is described [here](https://rgbds.gbdev.io/docs/master/gbz80.7#DAA).
+
+But why do we increment A using `ADD 1` when `INC A` does the same but is more efficient? That's because `DAA` evaluates the carry flag (see the linked description), but unlike `ADD`, `INC` does not affect that flag. So if the carry flag was still set from a previous operation<!-- note: this cannot happen right now, since IncreaseScorePackedBCD is only ever called after CP set the zero flag, which excludes setting the carry flag -->, `DAA` would add 60 points.
 
-Then we store the score back into `wScore` and finally, we call a function that will update the score board, which we will implement next.
+Now that score (in A) has been properly incremented, we store it back into `wScore` and finally, we call a function that will update the score board, which we will implement next.
 
 Of course, we still need to call it on impact. To do this, we add a call to `IncreaseScorePackedBCD` after each collision handler (we had a left and a right collision) in `CheckAndHandleBrick`
 
@@ -103,4 +107,4 @@ We then add the `DIGIT_OFFSET` constant to the tens digit to calculate the tile
 
 Finally, we repeat the process for the ones digit: We mask the tens digit from `A` using `and %00001111`, no need to rotate this time.
 
-Now we can display the score on the screen! We'll need to call `UpdateScoreBoard` after each time the score is updated. We've already done this in the `IncreaseScorePackedBCD` function, so we're all set!
+Now we can display the score on the screen! We'll need to call `UpdateScoreBoard` after each time the score is updated. We've already done this in the `IncreaseScorePackedBCD` function, so we're all set!

unbricked/bcd/main.asm

@@ -294,11 +294,10 @@ IsWallTile:
 ; Increase score by 1 and store it as a 1 byte packed BCD number
 ; changes A and HL
 IncreaseScorePackedBCD:
-    xor a               ; clear carry flag and a
-    inc a               ; a = 1
-    ld hl, wScore       ; load score
-    adc [hl]            ; add 1
-    daa                 ; convert to BCD
+    ld hl, wScore       ; load score address for faster access
+    ld a, [hl]          ; load score to accumulator
+    add 1
+    daa                 ; make sure it's a BCD
     ld [hl], a          ; store score
     call UpdateScoreBoard
     ret
@@ -754,4 +753,4 @@ wBallMomentumY: db
 ; ANCHOR: score-variable
 SECTION "Score", WRAM0
 wScore: db
-; ANCHOR_END: score-variable
+; ANCHOR_END: score-variable