diff options
Diffstat (limited to 'basics')
| -rw-r--r-- | basics/README.md | 15 | ||||
| -rw-r--r-- | basics/persist.s | 8 | ||||
| -rw-r--r-- | basics/sprite.s | 9 |
3 files changed, 23 insertions, 9 deletions
diff --git a/basics/README.md b/basics/README.md new file mode 100644 index 0000000..7571509 --- /dev/null +++ b/basics/README.md @@ -0,0 +1,15 @@ +## Basics + +Here we have simple programs which try to explain a particular topic of NES +programming. They are single files which already contain all you need in order +to have a working program. This means that there is quite a lot of boilerplate +you might not be aware. For this reason I encourage you to start with the +`sprite.s` file, which has a step by step explanation on how the NES is +initialized and what do all these magic "HEADER" and other jargon actually mean. +Anyhow, these programs are as follows: + +- `sprite.s`: detailed explanation on how to initialize the NES in order to have + some background and a sprite shown on screen. Follow along the code in order + to get a detailed explanation on each section. +- `input.s`: how to read the input from one controller. +- `persist.s`: using the MMC1 chip in order to persist data. diff --git a/basics/persist.s b/basics/persist.s index 1dcfa29..392797f 100644 --- a/basics/persist.s +++ b/basics/persist.s @@ -103,10 +103,10 @@ reset_mmc1: ;; That's why it did not really matter what we had at the iNES ;; header: mirroring is programmatically set, not ;; hardware-bound. - ;; - bit 2: we have two regions: $8000-$BFFF and $C000-$FFFF. This bit - ;; configures which one of them is fixed and which can be - ;; swapped. We set this bit to one, meaning that $8000-$BFFF is - ;; swappable while the other is fixed to the last bank of PRG. + ;; - bit 2: we have two regions (see bit 3): $8000-$BFFF and $C000-$FFFF. + ;; This bit configures which one of them is fixed and which can + ;; be swapped. We set this bit to one, meaning that $8000-$BFFF + ;; is swappable while the other is fixed to the last bank of PRG. ;; - bit 3: swappable PRG size. If set to 0, then 32KB of memory is ;; assumed, and thus bit 2 is ignored (i.e. the whole thing is to ;; be swapped at once). Otherwise, if set to 1, then 16KB is diff --git a/basics/sprite.s b/basics/sprite.s index ac68d7f..09dd8e5 100644 --- a/basics/sprite.s +++ b/basics/sprite.s @@ -49,9 +49,8 @@ ;; The previous are the mandatory bytes in order to get a "cartridge" going. ;; After this there are some other bytes you can put into the header, like - ;; if the "cartridge" counts on a battery-backed RAM-mapped section to save - ;; states (e.g. The Legend of Zelda) or rare flags like specifying the - ;; region (NTSC vs PAL), but for now this falls out of my radar :-) + ;; specifying the region (NTSC vs PAL), but for now this falls out of my + ;; radar :-) ;;; ;; This is the segment where we tell the processor where to find the code for @@ -422,8 +421,8 @@ palettes: ;; offset for the first element was $00C8, which added to the base address ;; of $2000 (start of the first nametable), gives us the address $20C8. ;; Therefore, if I want this background element to be rendered in the - ;; position I envisioned on this tool, I need to write $20C8 into PPUADDR - ;; (NOTE: these are a total of two bytes to be loaded, for operations that + ;; position I envisioned on this tool, I need to write $20C8 into PPUADDR. + ;; NOTE: these are a total of two bytes to be loaded, for operations that ;; only support one byte at a time. Hence, we have to load byte by byte (in ;; little-endian format) and store them. As for the PPUDATA address, I need ;; to pass #$02 because that's the index inside of the CHR file of the star |