new post

1 files changed, 95 insertions, 0 deletions

diff --git a/posts/writing-an-nes-game-part-3.md b/posts/writing-an-nes-game-part-3.md
new file mode 100644
index 0000000..998ae50
--- /dev/null
+++ b/posts/writing-an-nes-game-part-3.md
@@ -0,0 +1,95 @@
+---
+title: writing an nes game, part 3
+date: "2014-10-15 15:30"
+tags: [hacker school, nes]
+---
+
+# Writing an NES game - input handling
+
+Now that we have a general idea of how a program for the NES is structured,
+it'd be useful to get a bit further into the specific capabilities of the NES.
+One of the pretty basic (and pretty important) ones is reading input from the
+controllers.
+
+The basic structure of an NES controller is a [shift
+register](https://en.wikipedia.org/wiki/Shift_register). To read the
+controller inputs, you send a signal to the latch pin, which stores the state
+of the input buttons in the register, and then you send a sequence of signals
+to the clock pin to put the state of each button on the output in sequentially
+(A, B, Select, Start, Up, Down, Left, Right). As it turns out, the code you
+have to write to read from the controller maps pretty exactly to these
+operations. This is what it looks like:
+
+```asm
+read_controller1:
+  ; memory address $4016 corresponds to the shift register inside the
+  ; controller plugged into the first controller port. writing to it sets the
+  ; state of the latch pin, and so we set the latch pin high and then low in
+  ; order to store the controller state in the shift register.
+  LDA #$01
+  STA $4016
+  LDA #$00
+  STA $4016
+
+  ; reading from $4016 reads the output value of the data pin and also sends a
+  ; signal to the clock pin in order to put the next bit of data on the output
+  ; for the next read. the value that is read has the state of the button in
+  ; the low bit, and the upper bits contain various other pieces of
+  ; information (such as whether a controller is plugged in at all, etc), so
+  ; if we only care about the state of the button we have to mask out
+  ; everything else.
+read_a:
+  LDA $4016
+  AND #%00000001
+  BEQ read_b
+  ; code for if the a button is pressed
+read_b:
+  LDA $4016
+  AND #%00000001
+  BEQ read_select
+  ; code for if the b button is pressed
+read_select:
+  LDA $4016
+  AND #%00000001
+  BEQ read_start
+  ; code for if the select button is pressed
+read_start:
+  LDA $4016
+  AND #%00000001
+  BEQ read_up
+  ; code for if the start button is pressed
+read_up:
+  LDA $4016
+  AND #%00000001
+  BEQ read_down
+  ; code for if the up button is pressed
+read_down:
+  LDA $4016
+  AND #%00000001
+  BEQ read_left
+  ; code for if the down button is pressed
+read_left:
+  LDA $4016
+  AND #%00000001
+  BEQ read_right
+  ; code for if the left button is pressed
+read_right:
+  LDA $4016
+  AND #%00000001
+  BEQ end_read_controller1
+  ; code for if the right button is pressed
+
+end_read_controller1:
+  RTS
+```
+
+Obviously this could be simplified by putting the reads in a loop (the [snake
+game](https://github.com/doy/nes-snake) handles it by shifting and packing all
+of the button states into a single byte which the code can query later on),
+but that does require more CPU cycles, especially if not all of the buttons
+are important.
+
+In the spirit of continuing with real working code,
+[here]({{urls.media}}/input.nes) is a sample program which changes the
+background color every time you press A, rather than every second like last
+time. Tomorrow, we'll work on drawing sprites!