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| author | Miquel Sabaté Solà <mssola@mssola.com> | 2026-04-07 23:02:43 +0200 |
|---|---|---|
| committer | Miquel Sabaté Solà <mssola@mssola.com> | 2026-04-07 23:02:43 +0200 |
| commit | cd7e1e5514a6fd26ec0f01960807c6f2ab6f1a5b (patch) | |
| tree | 95e1fb041441341d6c778335858acaf70daf94fc | |
| parent | cdf9137af1d600c27bf0750615027859c9c2415c (diff) | |
| download | code.nes-cd7e1e5514a6fd26ec0f01960807c6f2ab6f1a5b.tar.gz code.nes-cd7e1e5514a6fd26ec0f01960807c6f2ab6f1a5b.zip | |
Expand the explanation on memory mapped IO
Signed-off-by: Miquel Sabaté Solà <mssola@mssola.com>
| -rw-r--r-- | basics/sprite.s | 23 |
1 files changed, 16 insertions, 7 deletions
diff --git a/basics/sprite.s b/basics/sprite.s index b841053..94cc0db 100644 --- a/basics/sprite.s +++ b/basics/sprite.s @@ -110,13 +110,22 @@ sei cld - ;; 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-$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 - ;; $4017 address controls what is called the "Frame counter" from the APU + ;; 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-$5FFF is reserved to I/O operations, where each address is reserved + ;; to a specific hardware operation. This is a wide range of addresses for a + ;; small set of registers that can actually be touched. But this is because + ;; on how the NES CPU is wired with the other chips and how "chip select" + ;; cables were layed out. I've found the following video useful to grasp + ;; this visually: https://youtu.be/lkJwpdw57lo?si=rNZhIzC3Gnf9ZU7A. In + ;; retrospect it feels like a big waste of addressing space, but this is how + ;; it is. + ;; + ;; Anyways, in this case ranges $4000-$4017 control the APU (Audio + ;; Processing Unit). More precisely, the $4017 address controls what is + ;; called the "Frame counter" from the APU ;; (https://www.nesdev.org/wiki/APU#Frame_Counter_($4017)). Setting #$40 to ;; it disables it completely, so we are in a known state. If we were to use ;; sound, at the end of the reset code we should enable it back. We do *not* |