Initial commit

Implemented the List scope which brings some handy macros and functions
to manipulate big lists on the NES.

Signed-off-by: Miquel Sabaté Solà <msabate@suse.com>

1 files changed, 81 insertions, 0 deletions

diff --git a/examples/mul.s b/examples/mul.s
new file mode 100644
index 0000000..81e12f4
--- /dev/null
+++ b/examples/mul.s
@@ -0,0 +1,81 @@
+;;;
+;; A silly example that tries to demonstrate how to use list.nes.
+;;
+;; It will initialize a list of 256 16-bit items with a value of 0..255
+;; respectively. After doing that, it will multiply each value by 2 and store it
+;; again. As a final touch, the second item will be forced to have the same
+;; value for the high and the low bytes.
+;;
+;; You can check that this works by running `make example` and then running the
+;; resulting ROM file into an emulator with a RAM inspector like FCEUX or Mesen.
+;; Then go from memory address $0400 onwards and check that the results are as
+;; described.
+
+.include "../test/common.s"
+
+main:
+    LIST_INIT $0400
+
+    ;; We will store 256 16-bit items and $90 will keep track of it.
+    lda #0
+    sta $90
+@fill_loop:
+    ;; The 16-bit item will have the value of the current index as stored in
+    ;; $90. The list will be initialized through the `List::push` function, so
+    ;; the size is properly set at the end.
+    lda $90
+    jsr List::push
+    lda #0
+    jsr List::push
+
+    ;; If we have set all the items we wanted, move into the next thing,
+    ;; otherwise increase $90 and go back into the loop.
+    lda $90
+    cmp #$FF
+    beq @set
+    inc $90
+    jmp @fill_loop
+
+@set:
+    ;; Reset the pointer to the start of the list since we want to iterate it
+    ;; over.
+    LIST_IT_FROM $0400
+
+@set_loop:
+    ;; There are two values to keep track: `List::ptr` and `List::ptr + 1`. For
+    ;; the low byte we need to shift left once, and then the high byte needs to
+    ;; add the possible carry to itself. In order to do this and call
+    ;; `List::set` properly we need to be do some stuff with the stack (low
+    ;; byte) or the `x` register (high byte).
+    ldy #0
+    lda (List::ptr), y
+    asl
+    pha
+
+    lda #0
+    iny
+    adc (List::ptr), y
+    tax
+
+    ;; Pull the value that corresponds to the low byte and set it. After doing
+    ;; that, though, did we get a $FF value? If so then it means that we are
+    ;; done with it, otherwise proceed to grab the high byte stored on the `x`
+    ;; registerm set it with `List::set` and loop again.
+    pla
+    jsr List::set
+    cpy #$FF
+    beq @final_get
+    txa
+    jsr List::set
+    jmp @set_loop
+
+@final_get:
+    ;; We are done! Now for a final touch let's copy the value from the low byte
+    ;; into the high byte of the second element.
+    LIST_IT_FROM $0402
+
+    jsr List::get
+    jsr List::set
+
+done:
+    jmp done