added programming!

programming.md

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+## programming george
+
+reading garth wilson's [stack treatise](https://wilsonminesco.com/stacks), the idea of having a data stack seperate from the hardware stack sounds cool. starting this page to keep track of the design decisions i'm making for how george should be programmed.
+
+### init
+
+### main loop
+
+### data stack
+
+putting a data stack on the zero page lets us use zero page indexed and zero page preindexed indirect instructions to interact with our subroutine parameters. that's a lot of words, but it just means that when we push/pull the stack, we use fewer cycles than if our stack was higher up in memory. this comes at the cost of our x register, which is now our data stack pointer.
+
+to push a byte onto the data stack, we just:
+
+```asm
+   dex            ; decrement the stack pointer
+   lda some_value ; load the byte we want on the stack into a
+   sta 0, x       ; put the byte on the stack!
+```
+
+and to pop a byte off it:
+
+```asm
+   lda 0, x       ; pop the top of stack off into a
+   inx            ; increment the stack pointer
+```
+
+### screen memory
+
+#### character mode
+
+characters are 8 dots wide by 13 scanlines high. the display is 512 dots wide by 380 scanlines high, so that's 512/8=64 characters per row and 380/13~=29 characters per column. we don't have to worry about how each individual bit maps to the screen, since vram is just a series of ascii character codes. the vram address for a character at a given X,Y coordinate will be `$6000 + (64\*Y) + X`.
+
+but i wrote a subroutine to do this for you! just push the coordinates on the data stack (x first, then y) and call `jsr get_char_address`, and the memory address will be put at the top of the stack.
+
+#### hires mode
+
+### misc notes
+
+putting this here cause i get it mixed up a lot: "low byte first" means lower in memory! so to put the word `$4a6f` on the stack, do this:
+
+```asm
+push ; macro for 2x dex
+lda #$6f
+sta 0, x
+lda #$4a
+sta 1, x
+```
+
+this means you can use the (zp, x) addressing mode to point at an address! using the same example, to store `$23` at the address `$4a6f` (which is on the stack), do this:
+
+```asm
+lda #$23
+sta (0, x)
+```