Subversion Repositories neorv32

:sectnums:

== Software Framework

To make actual use of the NEORV32 processor, the project comes with a complete software ecosystem. This

ecosystem is based on the RISC-V port of the GCC GNU Compiler Collection and consists of the following elementary parts:

* <<_compiler_toolchain>>

* <<_core_libraries>>

* <<_application_makefile>>

* <<_executable_image_format>>

** <<_linker_script>>

** <<_ram_layout>>

** <<_c_standard_library>>

** <<_start_up_code_crt0>>

* <<_bootloader>>

* <<_neorv32_runtime_environment>>

A summarizing list of the most important elements of the software framework and their according

files and folders is shown below:

[cols="<6,<4"]

[grid="none"]

|=======================

| Application start-up code               | `sw/common/crt0.S`

| Application linker script               | `sw/common/neorv32.ld`

| Core hardware driver libraries ("HAL")  | `sw/lib/include/` & `sw/lib/source/`

| Central application makefile            | `sw/common/common.mk`

| Tool for generating NEORV32 executables | `sw/image_gen/`

| Default bootloader                      | `sw/bootloader/bootloader.c`

| Example programs                        | `sw/example`

|=======================

.Software Documentation

[TIP]

All core libraries and example programs are highly documented using **Doxygen**.

See section <>.

The documentation is automatically built and deployed to GitHub pages and is available online

at https://stnolting.github.io/neorv32/sw/files.html .

// ####################################################################################################################

:sectnums:

=== Compiler Toolchain

The toolchain for this project is based on the free RISC-V GCC-port. You can find the compiler sources and

build instructions on the official RISC-V GNU toolchain GitHub page: https://github.com/riscv/riscv-gnutoolchain.

The NEORV32 implements a 32-bit RISC-V architecture and uses a 32-bit integer and soft-float ABI by default.

Make sure the toolchain / toolchain build is configured accordingly.

* MARCH = `rv32i`

* MABI = `ilp32`

Alternatively, you can download my prebuilt `rv32i/e` toolchains for 64-bit x86 Linux from: https://github.com/stnolting/riscv-gcc-prebuilt

The default toolchain prefix used by the project's makefiles is **`riscv32-unknown-elf`**, which can be changes

using makefile flags at any time.

[TIP]

More information regarding the toolchain (building from scratch or downloading the prebuilt ones)

can be found in the user guides' section https://stnolting.github.io/neorv32/ug/#_software_toolchain_setup[Software Toolchain Setup].

<<<

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=== Core Libraries

The NEORV32 project provides a set of C libraries that allows an easy usage of the processor/CPU features

(also called "HAL" - hardware abstraction layer). All driver and runtime-related files are located in

`sw/lib`. These are automatically included and linked by adding the following _include statement_:

[source,c]

----

#include  // add NEORV32 HAL and runtime libraries

----

[cols="<3,<4,<8"]

[options="header",grid="rows"]

|=======================

| C source file | C header file | Description

| -                   | `neorv32.h`            | main NEORV32 definitions and library file

| `neorv32_cfs.c`     | `neorv32_cfs.h`        | HW driver (stubs) functions for the custom functions subsystem

footnote:[This driver file only represents a stub, since the real CFS drivers are defined by the actual CFS implementation.]

| `neorv32_cpu.c`     | `neorv32_cpu.h`        | HW driver functions for the NEORV32 **CPU**

| `neorv32_cpu_cfu.c` | `neorv32_cpu_cfu.h`    | HW driver functions for the NEORV32 **CFU** (custom instructions)

| `neorv32_gpio.c`    | `neorv32_gpio.h`       | HW driver functions for the **GPIO**

| `neorv32_gptmr.c`   | `neorv32_gptmr.h`      | HW driver functions for the **GPTRM**

| -                   | `neorv32_intrinsics.h` | macros for custom intrinsics & instructions

| `neorv32_mtime.c`   | `neorv32_mtime.h`      | HW driver functions for the **MTIME**

| `neorv32_neoled.c`  | `neorv32_neoled.h`     | HW driver functions for the **NEOLED**

| `neorv32_pwm.c`     | `neorv32_pwm.h`        | HW driver functions for the **PWM**

| `neorv32_rte.c`     | `neorv32_rte.h`        | NEORV32 **runtime environment** and helper functions

| `neorv32_slink.c`   | `neorv32_slink.h`      | HW driver functions for the **SLINK**

| `neorv32_spi.c`     | `neorv32_spi.h`        | HW driver functions for the **SPI**

| `neorv32_trng.c`    | `neorv32_trng.h`       | HW driver functions for the **TRNG**

| `neorv32_twi.c`     | `neorv32_twi.h`        | HW driver functions for the **TWI**

| `neorv32_uart.c`    | `neorv32_uart.h`       | HW driver functions for the **UART0** and **UART1**

| `neorv32_wdt.c`     | `neorv32_wdt.h`        | HW driver functions for the **WDT**

| `neorv32_xip.c`     | `neorv32_xip.h`        | HW driver functions for the **XIP**

| `neorv32_xirq.c`    | `neorv32_xirq.h`       | HW driver functions for the **XIRQ**

| `syscalls.c`        | -                      | newlib system calls

|=======================

[TIP]

A CMSIS-SVD-compatible **System View Description (SVD)** file including all peripherals is available in `sw/svd`.

`sw/lib/include`. Currently, the following library files are available:

<<<

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:sectnums:

=== Application Makefile

Application compilation is based on a single, centralized **GNU makefiles** `sw/common/common.mk`. Each project in the

`sw/example` folder features a makefile that just includes this central makefile. When creating a new project copy an

existing project folder or at least the makefile to the new project folder. It is suggested to create new projects also

in `sw/example` to keep the file dependencies. However, these dependencies can be manually configured via makefiles

variables when the new project is located somewhere else.

[NOTE]

Before the makefile can be used to compile applications, the RISC-V GCC toolchain needs to be installed. Furthermore,

the `bin` folder of the compiler needs to be added to the system's `PATH` variable. More information can be found in

https://stnolting.github.io/neorv32/ug/#_software_toolchain_setup[User Guide: Software Toolchain Setup].

The makefile is invoked by simply executing `make` in the console. For example:

[source,bash]

----

neorv32/sw/example/blink_led$ make

----

:sectnums:

==== Targets

Just executing `make` (or executing `make help`) will show the help menu listing all available targets.

[source,makefile]

----

$ make

<<< NEORV32 SW Application Makefile >>>

Make sure to add the bin folder of RISC-V GCC to your PATH variable.

== Targets ==

 help         - show this text

 check        - check toolchain

 info         - show makefile/toolchain configuration

 exe          - compile and generate  executable for upload via bootloader

 hex          - compile and generate  executable raw file

 image        - compile and generate VHDL IMEM boot image (for application) in local folder

 install      - compile, generate and install VHDL IMEM boot image (for application)

 sim          - in-console simulation using default/simple testbench and GHDL

 all          - exe + hex + install

 elf_info     - show ELF layout info

 clean        - clean up project

 clean_all    - clean up project, core libraries and image generator

 bl_image     - compile and generate VHDL BOOTROM boot image (for bootloader only!) in local folder

 bootloader   - compile, generate and install VHDL BOOTROM boot image (for bootloader only!)

== Variables ==

 USER_FLAGS   - Custom toolchain flags [append only], default ""

 EFFORT       - Optimization level, default "-Os"

 MARCH        - Machine architecture, default "rv32i"

 MABI         - Machine binary interface, default "ilp32"

 APP_INC      - C include folder(s) [append only], default "-I ."

 ASM_INC      - ASM include folder(s) [append only], default "-I ."

 RISCV_PREFIX - Toolchain prefix, default "riscv32-unknown-elf-"

 NEORV32_HOME - NEORV32 home folder, default "../../.."

----

:sectnums:

==== Configuration

The compilation flow is configured via variables right at the beginning of the central

makefile (`sw/common/common.mk`):

[TIP]

The makefile configuration variables can be overridden or extended directly when invoking the makefile. For

example `$ make MARCH=rv32ic clean_all exe` overrides the default `MARCH` variable definitions.

Permanent modifications/definitions can be made in the project-local makefile

(e.g., `sw/example/blink_led/makefile`).

.Default Makefile Configuration

[source,makefile]

----

# *****************************************************************************

# USER CONFIGURATION

# *****************************************************************************

# User's application sources (*.c, *.cpp, *.s, *.S); add additional files here

APP_SRC ?= $(wildcard ./*.c) $(wildcard ./*.s) $(wildcard ./*.cpp) $(wildcard ./*.S)

# User's application include folders (don't forget the '-I' before each entry)

APP_INC ?= -I .

# User's application include folders - for assembly files only (don't forget the '-I' before each

entry)

ASM_INC ?= -I .

# Optimization

EFFORT ?= -Os

# Compiler toolchain

RISCV_PREFIX ?= riscv32-unknown-elf-

# CPU architecture and ABI

MARCH ?= rv32i

MABI  ?= ilp32

# User flags for additional configuration (will be added to compiler flags)

USER_FLAGS ?=

# Relative or absolute path to the NEORV32 home folder

NEORV32_HOME ?= ../../..

# *****************************************************************************

----

.Variables Description

[cols="<3,<10"]

[grid="none"]

|=======================

| `APP_SRC`      | The source files of the application (`*.c`, `*.cpp`, `*.S` and `*.s` files are allowed;

files of these types in the project folder are automatically added via wild cards). Additional files can be added separated by white spaces

| `APP_INC`      | Include file folders; separated by white spaces; must be defined with `-I` prefix

| `ASM_INC`      | Include file folders that are used only for the assembly source files (`*.S`/`*.s`).

| `EFFORT`       | Optimization level, optimize for size (`-Os`) is default; legal values: `-O0`, `-O1`, `-O2`, `-O3`, `-Os`, `-Ofast`, ...

| `RISCV_PREFIX` | The toolchain prefix to be used; follows the triplet naming convention `[architecture]-[host_system]-[output]-...`

| `MARCH`        | The targeted RISC-V architecture/ISA; enable compiler support of optional CPU extension by adding the according extension

name (e.g. `rv32im` for `M` CPU extension; see https://stnolting.github.io/neorv32/ug/#_enabling_risc_v_cpu_extensions[User Guide: Enabling RISC-V CPU Extensions]

for more information

| `MABI`         | Application binary interface (default: 32-bit integer ABI `ilp32`)

| `USER_FLAGS`   | Additional flags that will be forwarded to the compiler tools

| `NEORV32_HOME` | Relative or absolute path to the NEORV32 project home folder; adapt this if the makefile/project is not in the project's

default `sw/example` folder

|=======================

:sectnums:

==== Default Compiler Flags

The following default compiler flags are used for compiling an application. These flags are defined via the

`CC_OPTS` variable. Custom flags can be _appended_ to it using the `USER_FLAGS` variable.

[cols="<3,<9"]

[grid="none"]

|=======================

| `-Wall`                  | Enable all compiler warnings.

| `-ffunction-sections`    | Put functions and data segment in independent sections. This allows a code optimization as dead code and unused data can be easily removed.

| `-nostartfiles`          | Do not use the default start code. Instead, use the NEORV32-specific start-up code (`sw/common/crt0.S`).

| `-Wl,--gc-sections`      | Make the linker perform dead code elimination.

| `-lm`                    | Include/link with `math.h`.

| `-lc`                    | Search for the standard C library when linking.

| `-lgcc`                  | Make sure we have no unresolved references to internal GCC library subroutines.

| `-mno-fdiv`              | Use built-in software functions for floating-point divisions and square roots (since the according instructions are not supported yet).

| `-falign-functions=4` .4+| Force a 32-bit alignment of functions and labels (branch/jump/call targets). This increases performance as it simplifies instruction fetch when using the C extension. As a drawback this will also slightly increase the program code.

| `-falign-labels=4`

| `-falign-loops=4`

| `-falign-jumps=4`

|=======================

<<<

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:sectnums:

=== Executable Image Format

In order to generate a file, which can be executed by the processor, all source files have to be compiler, linked

and packed into a final _executable_.

:sectnums:

==== Linker Script

When all the application sources have been compiled, they need to be _linked_ in order to generate a unified

program file. For this purpose the makefile uses the NEORV32-specific linker script `sw/common/neorv32.ld` for

linking all object files that were generated during compilation.

The linker script defines three memory _sections_: `rom`, `ram` and `iodev`. Each section provides specific

access _attributes_: read access (`r`), write access (`w`) and executable (`x`).

.Linker memory sections - general

[cols="<2,^1,<7"]

[options="header",grid="rows"]

|=======================

| Memory section  | Attributes | Description

| `ram`           | `rwx`      | Data memory address space (processor-internal/external DMEM)

| `rom`           | `rx`       | Instruction memory address space (processor-internal/external IMEM) _or_ internal bootloader ROM

| `iodev`         | `rw`       | Processor-internal memory-mapped IO/peripheral devices address space

|=======================

These sections are defined right at the beginning of the linker script:

.Linker memory sections - cut-out from linker script `neorv32.ld`

[source,c]

----

MEMORY

{

  ram  (rwx) : ORIGIN = 0x80000000, LENGTH = DEFINED(make_bootloader) ? 512 : 8*1024

  rom   (rx) : ORIGIN = DEFINED(make_bootloader) ? 0xFFFF0000 : 0x00000000, LENGTH = DEFINED(make_bootloader) ? 32K : 2048M

  iodev (rw) : ORIGIN = 0xFFFFFE00, LENGTH = 512

}

----

Each memory section provides a _base address_ `ORIGIN` and a _size_ `LENGTH`. The base address and size of the `iodev` section is

fixed and should not be altered. The base addresses and sizes of the `ram` and `rom` regions correspond to the total available instruction

and data memory address space (see section <<_address_space_layout>>) as defined in `rtl/core/neorv32_package.vhd`.

[IMPORTANT]

`ORIGIN` of the `ram` section has to be always identical to the processor's `dspace_base_c` hardware configuration. +

 +

`ORIGIN` of the `rom` section has to be always identical to the processor's `ispace_base_c` hardware configuration.

The sizes of `rom` section is a little bit more complicated. The default linker script configuration assumes a _maximum_ of 2GB _logical_

memory space, which is also the default configuration of the processor's hardware instruction memory address space. This size does not have

to reflect the _actual_ physical size of the instruction memory (internal IMEM and/or processor-external memory). It just provides a maximum

limit. When uploading new executable via the bootloader, the bootloader itself checks if sufficient _physical_ instruction memory is available.

If a new executable is embedded right into the internal-IMEM the synthesis tool will check, if the configured instruction memory size

is sufficient (e.g., via the <<_mem_int_imem_size>> generic).

[IMPORTANT]

The `rom` region uses a conditional assignment (via the `make_bootloader` symbol) for `ORIGIN` and `LENGTH` that is used to place

"normal executable" (i.e. for the IMEM) or "the bootloader image" to their according memories. +

 +

The `ram` region also uses a conditional assignment (via the `make_bootloader` symbol) for `LENGTH`. When compiling the bootloader

(`make_bootloader` symbol is set) the generated bootloader will only use the _first_ 512 bytes of the data address space. This is

a fall-back to ensure the bootloader can operate independently of the actual _physical_ data memory size.

The linker maps all the regions from the compiled object files into five final sections: `.text`, `.rodata`, `.data`, `.bss` and `.heap`.

These regions contain everything required for the application to run:

.Linker memory regions

[cols="<1,<9"]

[options="header",grid="rows"]

|=======================

| Region    | Description

| `.text`   | Executable instructions generated from the start-up code and all application sources.

| `.rodata` | Constants (like strings) from the application; also the initial data for initialized variables.

| `.data`   | This section is required for the address generation of fixed (= global) variables only.

| `.bss`    | This section is required for the address generation of dynamic memory constructs only.

| `.heap`   | This section is required for the address generation of dynamic memory constructs only.

|=======================

The `.text` and `.rodata` sections are mapped to processor's instruction memory space and the `.data`,

`.bss` and `heap` sections are mapped to the processor's data memory space. Finally, the `.text`, `.rodata` and `.data`

sections are extracted and concatenated into a single file `main.bin`.

:sectnums:

==== RAM Layout

The default NEORV32 linker script uses all of the defined RAM (linker script memory section `ram`) to create four areas.

Note that depending on the application some areas might not be existent at all.

.Default RAM Layout

image::ram_layout.png[400]

[start=1]

. **Constant data (`.data`)**: The constant data section is placed right at the beginning of the RAM. For example, this section

contains _explicitly initialized_ global variables. This section is initialized by the executable.

. **Dynamic data (`.bss`)**: The constant data section is followed by the dynamic data section, which contains _uninitialized_ data

like global variables without explicit initialization. This section is cleared by the start-up code `crt0.S`.

. **Heap (`.heap`)**: The heap is used for dynamic memory that is managed by functions like `malloc()` and `free()`. The heap

grows upwards. This section is not initialized at all.

. **Stack**: The stack starts at the very end of the RAM at address `ORIGIN(ram) + LENGTH(ram) - 4`. The stack grows downwards.

There is _no explicit limit_ for the maximum stack size as this is hard to check. However, a physical memory protection rule could

be used to configure a maximum size by adding a "protection area" between stack and heap (a PMP region without any access rights).

The maximum size of the heap is defined by the linker script's `__heap_size` symbol. This symbol can be overridden at any time.

By default, the maximum heap size is 1/4 of the total RAM size.

.Heap-Stack Collisions

[WARNING]

Take care when using dynamic memory to avoid collision of the heap and stack memory areas. There is no compile-time protection

mechanism available as the actual heap and stack size are defined by _runtime_ data. Also beware of fragmentation when

using dynamic memory allocation.

:sectnums:

==== C Standard Library

.Constructors and Deconstructors

[IMPORTANT]

The NEORV32 processor is an embedded system intended for running bare-metal or RTOS applications. To simplify this setup

explicit constructors and deconstructors are not supported by default. However, a minimal "deconstructor-alike" support is

provided by the <<_after_main_handler>>.

The NEORV32 is a processor for _embedded_ applications. Hence, it is not capable of running desktop OSs like Linux

(at least not without emulation). Hence, the software framework relies on a "bare-metal" setup that uses **newlib**

as default C standard library.

.RTOS Support

[NOTE]

The NEORV32 CPU and processor **do support** embedded RTOS like FreeRTOS and Zephyr. See the User guide section

https://stnolting.github.io/neorv32/ug/#_zephyr_rtos_support[Zephyr RTOS Support] and

https://stnolting.github.io/neorv32/ug/#_freertos_support[FreeRTOS Support]

for more information.

Newlib provides stubs for common "system calls" (like file handling and standard input/output) that are used by other

C libraries like `stdio`. These stubs are available in `sw/source/syscalls.c` and were adapted for the NEORV32 processor.

.Standard Console(s)

[NOTE]

<<_primary_universal_asynchronous_receiver_and_transmitter_uart0, UART0>>

is used to implement all the standard input, output and error consoles (`STDIN`, `STDOUT` and `STDERR`).

.Newlib Test/Demo Program

[TIP]

A simple test and demo program, which uses some of newlib's core functions (like `malloc`/`free` and `read`/`write`)

is available in `sw/example_newlib_demo`

:sectnums:

==== Executable Image Generator

The `main.bin` file is packed by the NEORV32 image generator (`sw/image_gen`) to generate the final executable file.

[NOTE]

The sources of the image generator are automatically compiled when invoking the makefile.

The image generator can generate three types of executables, selected by a flag when calling the generator:

[cols="<1,<9"]

[grid="none"]

|=======================

| `-app_bin` | Generates an executable binary file `neorv32_exe.bin` (for UART uploading via the bootloader).

| `-app_hex` | Generates a plain ASCII hex-char file `neorv32_exe.hex` that can be used to initialize custom (instruction-) memories (in synthesis/simulation).

| `-app_img` | Generates an executable VHDL memory initialization image for the processor-internal IMEM. This option generates the `rtl/core/neorv32_application_image.vhd` file.

| `-bld_img` | Generates an executable VHDL memory initialization image for the processor-internal BOOT ROM. This option generates the `rtl/core/neorv32_bootloader_image.vhd` file.

|=======================

All these options are managed by the makefile. The _normal application_ compilation flow will generate the `neorv32_exe.bin`

executable to be upload via UART to the NEORV32 bootloader.

The image generator add a small header to the `neorv32_exe.bin` executable, which consists of three 32-bit words located right at the

beginning of the file. The first word of the executable is the signature word and is always `0x4788cafe`. Based on this word the bootloader

can identify a valid image file. The next word represents the size in bytes of the actual program

image in bytes. A simple "complement" checksum of the actual program image is given by the third word. This

provides a simple protection against data transmission or storage errors.

:sectnums:

==== Start-Up Code (crt0)

The CPU and also the processor require a minimal start-up and initialization code to bring the CPU (and the SoC)

into a stable and initialized state and to initialize the C runtime environment before the actual application can be executed.

This start-up code is located in `sw/common/crt0.S` and is automatically linked _every_ application program

and placed right before the actual application code so it gets executed right after reset.

The `crt0.S` start-up performs the following operations:

[start=1]

. Initialize all integer registers `x1 - x31` (or jsut `x1 - x15` when using the `E` CPU extension) to a defined value.

. Initialize the global pointer `gp` and the stack pointer `sp` according to the `.data` segment layout provided by the linker script.

. Initialize all CPU core CSRs and also install a default "dummy" trap handler for _all_ traps. This handler catches all traps during the early boot phase.

. Clear IO area: Write zero to all memory-mapped registers within the IO region (`iodev` section). If certain devices have not been implemented, a bus access fault exception will occur. This exception is captured by the dummy trap handler.

. Clear the `.bss` section defined by the linker script.

. Copy read-only data from the `.text` section to the `.data` section to set initialized variables.

. Call the application's `main` function (with _no_ arguments: `argc` = `argv` = 0).

. If the `main` function returns `crt0` can call an "after-main handler" (see below)

. If there is no after-main handler or after returning from the after-main handler the processor goes to an endless sleep mode (using a simple loop or via the `wfi` instruction if available).

:sectnums:

===== After-Main Handler

If the application's `main()` function actually returns, an _after main handler_ can be executed. This handler can be a normal function

since the C runtime is still available when executed. If this handler uses any kind of peripheral/IO modules make sure these are

already initialized within the application or you have to initialize them _inside_ the handler.

.After-main handler - function prototype

[source,c]

----

int __neorv32_crt0_after_main(int32_t return_code);

----

The function has exactly one argument (`return_code`) that provides the _return value_ of the application's main function.

For instance, this variable contains _-1_ if the main function returned with `return -1;`. The return value of the

`__neorv32_crt0_after_main` function is irrelevant as there is no further "software instance" executed afterwards that can check this.

However, the on-chip debugger could still evaluate the return value of the after-main handler.

A simple UARt output can be used to inform the user when the application's main function returns

(this example assumes that UART0 has been already properly configured in the actual application):

.After-main handler - simple example

[source,c]

----

int __neorv32_crt0_after_main(int32_t return_code) {

  neorv32_uart0_printf("\n main function returned with exit code %i. \n", return_code); <1>

  return 0;

}

----

<1> Use `` here to make clear this is a message comes from the runtime environment.

<<<

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include::software_bootloader.adoc[]

<<<

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include::software_rte.adoc[]