Fix dead link

Author: twilco

_posts/2019-04-27-riscv-from-scratch-2.markdown

@@ -248,7 +248,7 @@ Using `qemu` and `dtc`, we've successfully discovered where the RAM lives and ho
 
 Well, since the default `crt0` isn't doing what we need it to, we have one obvious choice: writing our own, and then linking it with the object file created from compiling our simple addition program.  Our `crt0` will need to know where the top of the stack starts in order to properly initialize it.  We could hardcode this value to `0x80000000` directly in our `crt0`, but that isn't a very maintainable solution.  What happens when we want to use a different `qemu`-lated CPU, such as the `sifive_e`, that has different memory properties?  
 
-Fortunately for us, we are far from the first to ask this question, and a good solution exists.  GNU's linking program, `ld`, [provides a way for us to define a symbol](https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/4/html/Using_ld_the_GNU_Linker/assignments.html) which would be accessible from our `crt0`.  We can use this, among other functions provided by `ld`, to create a `__stack_top` symbol definition that is reasonably flexible across multiple different CPUs.
+Fortunately for us, we are far from the first to ask this question, and a good solution exists.  GNU's linking program, `ld`, [provides a way for us to define a symbol](https://web.archive.org/web/20190525173911/https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/4/html/Using_ld_the_GNU_Linker/assignments.html) which would be accessible from our `crt0`.  We can use this, among other functions provided by `ld`, to create a `__stack_top` symbol definition that is reasonably flexible across multiple different CPUs.
 
 Rather than writing our own linker file from scratch, it is going to make more sense to take the default linker script that `ld` uses and modify it slightly to expose any additional symbols we want. What is a linker script, you might be wondering?  [This snippet summarizes it well:](http://www.scoberlin.de/content/media/http/informatik/gcc_docs/ld_3.html)
 
@@ -317,7 +317,7 @@ SECTIONS
   .note.gnu.build-id  : { *(.note.gnu.build-id) }
 {% endhighlight %}
 
-As you can see, we use the [PROVIDE command](https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/4/html/Using_ld_the_GNU_Linker/assignments.html#PROVIDE) to define a symbol called `__stack_top`.  `__stack_top` will be accessible from any program linked with this script (assuming the program itself does not also define something named `__stack_top`).  We set the value of `__stack_top` to be `ORIGIN(RAM)`, which we know is `0x80000000`, plus `LENGTH(RAM)`, which we know is 128 megabytes (`0x8000000` bytes).  This means our `__stack_top` is set to `0x88000000`.
+As you can see, we use the [PROVIDE command](https://web.archive.org/web/20190525173911/https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/4/html/Using_ld_the_GNU_Linker/assignments.html#PROVIDE) to define a symbol called `__stack_top`.  `__stack_top` will be accessible from any program linked with this script (assuming the program itself does not also define something named `__stack_top`).  We set the value of `__stack_top` to be `ORIGIN(RAM)`, which we know is `0x80000000`, plus `LENGTH(RAM)`, which we know is 128 megabytes (`0x8000000` bytes).  This means our `__stack_top` is set to `0x88000000`.
 
 ### Stop!  <s>Hammertime</s> Runtime!
 <div style="margin-top: -30px; margin-bottom: 10px;"><sub><sup><sub><sup><a href="https://www.youtube.com/watch?v=otCpCn0l4Wo">https://www.youtube.com/watch?v=otCpCn0l4Wo</a></sup></sub></sup></sub></div>