diff options
| -rw-r--r-- | basics/sprite.s | 41 |
1 files changed, 28 insertions, 13 deletions
diff --git a/basics/sprite.s b/basics/sprite.s index 6fcdad3..b9e183b 100644 --- a/basics/sprite.s +++ b/basics/sprite.s @@ -10,12 +10,11 @@ ;; through a linker configuration. You can provide a configuration of your own, ;; but bear in mind that compilers like `cc65` (the one used here, which is the ;; most common) already provide a default configuration for the linker that -;; glues a set of pretty common defined named segments. You can read about this -;; in `cfg/nes.cfg` from inside your cc65 installation. Otherwise, I have also -;; written linker configuration files for other examples: take a look, for +;; glues a set of pretty commonly defined named segments. You can read about +;; this in `cfg/nes.cfg` from inside your cc65 installation. Otherwise, I have +;; also written linker configuration files for other examples: take a look, for ;; instance, at the `config/unrom.cfg` file, which is used by the ;; `basics/unrom.s` program. -;;; ;;; ;; The "HEADER" is the first segment of any iNES binary and it contains basic @@ -32,8 +31,10 @@ .byte 'N', 'E', 'S', $1A ;; The next two bytes define the size of the PRG and CHR ROMs in this order. - ;; Hence, the next two bytes define a 32KB (2 x 16KB) of PRG-ROM, and 8KB - ;; (1 x 8KB) of CHR-ROM. + ;; Hence, the next two bytes define a 32KB (2 x 16KB) of PRG-ROM, and 8KB (1 + ;; x 8KB) of CHR-ROM. This is the value expected on "NROM" cartridges (see + ;; https://www.nesdev.org/wiki/NROM), which were the cartridges first used + ;; by Nintendo (e.g. Super Mario Bros.). .byte $02 .byte $01 @@ -60,9 +61,10 @@ ;; handler, and the IRQ handler. If you look at the configuration from `cc65` ;; that was provided on your installation (or if you are pesky enough to create ;; one yourself), you will notice that these vector addresses are placed at the -;; very end of memory ($fffa-ffff). The NES (and emulators) will look at these -;; three last positions in memory to know where to jump for each case. -;;; +;; very end of memory ($fffa-ffff). This is not a quirk from the configuration, +;; but it's a requirement from the architecture of the 6502 processor. Hence, +;; the NES/Famicom (and emulators) will look at these three last addresses to +;; know where to jump for each case. .segment "VECTORS" .addr nmi .addr reset @@ -105,7 +107,7 @@ ;; Disable APU frame IRQ. This is the first instance we see of Memory-Mapped ;; I/O. This is a core concept in NES programming and, to sum things up, the - ;; memory range $2000-$6000 is reserved to I/O operations, and each address + ;; memory range $2000-$5FFF is reserved to I/O operations, and each address ;; is reserved to a specific hardware operation. This is because the NES CPU ;; doesn't directly control the PPU nor other chips. In this case, ranges ;; $4000-$4017 control the APU (Audio Processing Unit). More precisely, the @@ -117,9 +119,22 @@ ldx #$40 stx $4017 ; APU Frame Counter - ;; Set up the stack register with the proper value (the stack will grow in - ;; decreasing order from $01FF -> $0100). - ldx #$ff + ;; By the architecture of the 6502 processor, the stack is last-in-first-out + ;; (LIFO) and lives at the first page ($100-$1FF). This way the stack + ;; register can be an 8-bit register instead of a 16-bit one. Moreover, the + ;; stack grows top-down: from $1FF to $100. With all of this at hand, we can + ;; simply initialize this register to $FF, which has to be loaded first on + ;; the `x` register since `txs` (Transfer X to Stack Pointer) is the only + ;; instruction in standard 6502 assembler which can touch the stack pointer. + ;; + ;; As a side note, in more complex scenarios in which you want to squeeze as + ;; much RAM-space as possible, you might want to use the first page for + ;; other tasks if the stack is guaranteed to not grow too much. If you check + ;; `scroll/include/buffer.s`, you will see that in there the VRAM buffer is + ;; reserved to grow from the lower end of the first page. A strict limit + ;; must be guaranteed so neither the stack nor the VRAM buffer overstep each + ;; other. + ldx #$FF txs ;; And now disable, in this order, NMI, rendering and DMC IRQs. Note that |