Clean up build variants.

Neotron OS now only ever gets 4KiB of RAM at 0x2000_0000. It learns
about the rest from the BIOS. We always assume the BIOS is 128 KiB. If
it is larger, it has to split itself into two parts.

Author: thejpster

README.md

@@ -19,49 +19,38 @@ This OS is a work in progress. We intend to support:
 
 ## Build instructions
 
-Your board will need an appropriate Neotron BIOS installed, and you need to have OpenOCD running for your particular board. You also need to set the linker 
-arguments so you link the binary to suit the memory available on your system.
+Your board will need an appropriate Neotron BIOS installed, and you need to
+have OpenOCD (or other programming tool) running for your particular board.
+You may also need to set the linker arguments so you link the binary to suit
+the memory available on your system.
 
-### Build Instructions for the Neotron Pico
-
-The Neotron Pico has some special memory requirements - in particular, the
-flash lives at `0x1000_0000` and not `0x0000_0000`. There is 1920 KiB of
-flash, and 240 KiB of RAM available.
+### Build Instructions for the Neotron Pico (and other systems with Flash at `0x1000_0000`)
 
 ```
 $ git clone https://github.com/neotron-compute/Neotron-OS.git
 $ cd Neotron-OS
 $ git submodule update --init
-$ RUSTFLAGS="-C link-arg=-Tneotron-os-pico.ld" cargo build --release --target=thumbv6m-none-eabi
+$ RUSTFLAGS="-C link-arg=-Tneotron-flash-1000.ld" cargo build --release --target=thumbv6m-none-eabi
 ```
 
-### Build Instructions for 256K RAM systems
-
-Systems which reserve the second 512 KiB of Flash and first 256 KiB of SRAM
-for the OS can use this linker script. These systems include the Neotron
-340ST.
+### Build Instructions for the STM32 (and other systems with Flash at `0x0800_0000`)
 
 ```
 $ git clone https://github.com/neotron-compute/Neotron-OS.git
 $ cd Neotron-OS
 $ git submodule update --init
-$ RUSTFLAGS="-C link-arg=-Tneotron-os-256k.ld" cargo run --release
+$ RUSTFLAGS="-C link-arg=-Tneotron-flash-0800.ld" cargo build --release --target=thumbv6m-none-eabi
 ```
 
-### Build Instructions for 32K RAM systems
-
-Systems which reserve the second 128 KiB of Flash and first 26 KiB of SRAM for
-the OS can use this linker script. These systems include the Neotron 32.
+### Build Instructions for other systems (with Flash at `0x0000_0000`)
 
 ```
 $ git clone https://github.com/neotron-compute/Neotron-OS.git
 $ cd Neotron-OS
 $ git submodule update --init
-$ RUSTFLAGS="-C link-arg=-Tneotron-os-26k.ld" cargo run --release
+$ RUSTFLAGS="-C link-arg=-Tneotron-flash-0000.ld" cargo run --release
 ```
 
-TODO: Think of a better way of setting the memory limits for a particular OS build.
-
 ## Changelog
 
 ### Unreleased Changes ([Source](https://github.com/neotron-compute/Neotron-OS/tree/master))

neotron-os-26k.ld neotron-flash-0000.ldneotron-os-26k.ld renamed to neotron-flash-0000.ld

@@ -21,10 +21,13 @@
 /* Provides information about the memory layout of the device */
 MEMORY
 {
-    /* The OS gets the top 128 KiB of Flash, leaving the first 128 KiB for the BIOS */
+    /* The first 128 KiB is for the BIOS. We get the rest. */
     FLASH (rx)  : ORIGIN = 0x00020000, LENGTH = 128K
-    /* The BIOS gets the top 6 KiB of SRAM (including the Stack), leaving 26 KiB for the OS (at 0x2000_0000 to 0x2000_67FF) */
-    RAM   (rwx) : ORIGIN = 0x20000000, LENGTH = 26K
+    /*
+     * We get the bottom 4KB of RAM. Anything above that is for applications
+     * (up to wherever the BIOS tells us we can use.)
+     */
+    RAM   (rwx) : ORIGIN = 0x20000000, LENGTH = 4K
 }
 
 /* # Entry point = what the BIOS calls to start the OS */

neotron-os-256k.ld neotron-flash-0800.ldneotron-os-256k.ld renamed to neotron-flash-0800.ld

@@ -21,14 +21,13 @@
 /* Provides information about the memory layout of the device */
 MEMORY
 {
-    /* The BIOS gets the first 512 KiB of Flash, leaving 512 KiB for the OS */
-    FLASH (rx)  : ORIGIN = 0x08080000, LENGTH = 512K
-    /* The BIOS gets the top 64 KiB of SRAM (including the Stack), leaving 256 KiB for the OS
-       (at 0x2000_0000 to 0x2003_FFFF). The RAM is actually split into three banks, but we can
-       largely ignore that.
-    */
-    RAM   (rwx) : ORIGIN = 0x20000000, LENGTH = 256K
-    /* The SDRAM holds the LCD framebuffers */
+    /* The first 128 KiB is for the BIOS. We get the rest. */
+    FLASH (rx)  : ORIGIN = 0x08020000, LENGTH = 128K
+    /*
+     * We get the bottom 4KB of RAM. Anything above that is for applications
+     * (up to wherever the BIOS tells us we can use.)
+     */
+    RAM   (rwx) : ORIGIN = 0x20000000, LENGTH = 4K
 }
 
 /* # Entry point = what the BIOS calls to start the OS */

neotron-os-pico.ld neotron-flash-1000.ldneotron-os-pico.ld renamed to neotron-flash-1000.ld

@@ -21,15 +21,13 @@
 /* Provides information about the memory layout of the device */
 MEMORY
 {
-    /* The BIOS gets the first 128 KiB of Flash, leaving 1920 KiB for the OS */
-    FLASH (rx)  : ORIGIN = 0x10020000, LENGTH = 0x1E0000
+    /* The first 128 KiB is for the BIOS. We get the rest. */
+    FLASH (rx)  : ORIGIN = 0x10020000, LENGTH = 128K
     /*
-     * The RP2040 has 256 KiB of SRAM striped across four banks (for high
-     * performance), plus a fifth bank containing another 8 KiB of RAM. The
-     * BIOS is at the top of the high-performance RAM. We get the lower part.
+     * We get the bottom 4KB of RAM. Anything above that is for applications
+     * (up to wherever the BIOS tells us we can use.)
      */
-    RAM   (rwx) : ORIGIN = 0x20000000, LENGTH = 0x3C000
-    /* The SDRAM holds the LCD framebuffers */
+    RAM   (rwx) : ORIGIN = 0x20000000, LENGTH = 4K
 }
 
 /* # Entry point = what the BIOS calls to start the OS */

src/main.rs

@@ -295,7 +295,7 @@ extern "C" fn main(api: &'static bios::Api) -> ! {
         let mut width = 0;
         let mut height = 0;
         (api.video_memory_info_get)(&mut addr, &mut width, &mut height);
-        if addr != core::ptr::null_mut() {
+        if !addr.is_null() {
             let mut vga = VgaConsole {
                 addr,
                 width,