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== Enabling RISC-V CPU Extensions

Whenever you enable/disable a RISC-V CPU extensions via the according `CPU_EXTENSION_RISCV_x` generic, you need to
adapt the toolchain configuration so the compiler can actually generate according code for it.

To do so, open the makefile of your project (for example `sw/example/blink_led/makefile`) and scroll to the
"USER CONFIGURATION" section right at the beginning of the file. You need to modify the `MARCH` variable and eventually
the `MABI` variable according to your CPU hardware configuration.

[source,makefile]
----
# CPU architecture and ABI
MARCH ?= rv32i <1>
MABI  ?= ilp32 <2>
----
<1> MARCH = Machine architecture ("ISA string")
<2> MABI = Machine binary interface

For example, if you enable the RISC-V `C` extension (16-bit compressed instructions) via the `CPU_EXTENSION_RISCV_C`
generic (set `true`) you need to add the `c` extension also to the `MARCH` ISA string in order to make the compiler
emit compressed instructions.

.Privileged Architecture Extensions
[IMPORTANT]
Privileged architecture extensions like `Zicsr` or `Zifencei` are "used" _implicitly_ by the compiler. Hence, according
instruction will only be generated when "encoded" via inline assembly or when linking according libraries. In this case,
these instruction will _always_ be emitted (even if the according extension is not specified in `MARCH`). +
**I recommend to _not_ specify any privileged architecture extensions in `MARCH`.**

[WARNING]
ISA extension enabled in hardware can be a superset of the extensions enabled in software, but not the other way
around. For example generating compressed instructions for a CPU configuration that has the `c` extension disabled
will cause _illegal instruction exceptions_ at runtime.

You can also override the default `MARCH` and `MABI` configurations from the makefile when invoking the makefile:

[source,bash]
----
$ make MARCH=rv32ic clean_all all
----

[NOTE]
The RISC-V ISA string for `MARCH` follows a certain _canonical_ structure:
`rev32[i/e][m][a][f][d][g][q][c][b][v][n]...` For example `rv32imac` is valid while `rv32icma` is not.