Initial commit.

.gitignore

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+build/

LICENSE

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+Copyright (c) 2013, The Regents of the University of California (Regents).
+Copyright (c) 2018-2019, The libfemto authors
+All Rights Reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+1. Redistributions of source code must retain the above copyright
+   notice, this list of conditions and the following disclaimer.
+2. Redistributions in binary form must reproduce the above copyright
+   notice, this list of conditions and the following disclaimer in the
+   documentation and/or other materials provided with the distribution.
+3. Neither the name of the Regents nor the
+   names of its contributors may be used to endorse or promote products
+   derived from this software without specific prior written permission.
+
+IN NO EVENT SHALL REGENTS BE LIABLE TO ANY PARTY FOR DIRECT, INDIRECT,
+SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, INCLUDING LOST PROFITS, ARISING
+OUT OF THE USE OF THIS SOFTWARE AND ITS DOCUMENTATION, EVEN IF REGENTS HAS
+BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+REGENTS SPECIFICALLY DISCLAIMS ANY WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+PURPOSE. THE SOFTWARE AND ACCOMPANYING DOCUMENTATION, IF ANY, PROVIDED
+HEREUNDER IS PROVIDED "AS IS". REGENTS HAS NO OBLIGATION TO PROVIDE
+MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.

Makefile

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+CROSS_COMPILE      ?= riscv64-unknown-elf-
+
+AR                 = $(CROSS_COMPILE)ar
+
+CFLAGS             = -mcmodel=medany -ffunction-sections -fdata-sections
+LDFLAGS            = -nostartfiles -nostdlib -nostdinc -static -lgcc \
+                     -Wl,--nmagic -Wl,--gc-sections
+INCLUDES           = -Ienv/common
+
+libfemto_dirs      = libfemto/std libfemto/drivers libfemto/arch/riscv
+libfemto_src       = $(sort $(foreach d,$(libfemto_dirs),$(wildcard $(d)/*.c)))
+libfemto_asm       = $(sort $(foreach d,$(libfemto_dirs),$(wildcard $(d)/*.s)))
+libfemto_objs      = $(patsubst %.s,%.o,$(libfemto_asm)) \
+                     $(patsubst %.c,%.o,$(libfemto_src))
+
+#
+# Compiler configurations and target environment definitions
+#
+
+subdirs            = examples
+
+libs               = libfemto
+
+configs            = rv64imac
+
+CC_rv32imac        = $(CROSS_COMPILE)gcc
+CFLAGS_rv32imac    = -Os -march=rv32imac -mabi=ilp32 -Ienv/common/rv32
+LDFLAGS_rv32imac   =
+
+CC_rv64imac        = $(CROSS_COMPILE)gcc
+CFLAGS_rv64imac    = -Os -march=rv64imac -mabi=lp64  -Ienv/common/rv64
+LDFLAGS_rv64imac   =
+
+targets            = rv64imac:qemu-sifive_u
+
+#
+# make rules
+#
+
+all: all_programs
+
+clean:
+	rm -fr build
+
+backup: clean
+	tar czf ../$(shell basename $(shell pwd)).tar.gz .
+
+#
+# To view commands use: make V=1
+#
+
+ifdef V
+cmd = @mkdir -p $2 ; echo "$3"; $3
+else
+cmd = @echo "$1"; mkdir -p $2 ; $3
+endif
+
+#
+# Build system functions to generate pattern rules for all configs
+#
+
+define pattern =
+build/obj/$(2)/%.o: %.$(3)
+	$(call cmd,$(1).$(2) $$@,$$(@D),$(CC_$(2)) $(CFLAGS_$(2)) $(CFLAGS) \
+	$$(INCLUDES) -c $$^ -o $$@)
+endef
+
+$(foreach c,$(configs),$(eval $(call pattern,CC,$(c),c)))
+$(foreach c,$(configs),$(eval $(call pattern,AS,$(c),s)))
+
+#
+# Build system functions to generate library rules for all configs
+#
+
+define archive =
+build/lib/$(2)/$(3).a: $(addprefix build/obj/$(2)/,$($(3)_objs))
+	$(call cmd,$(1).$(2) $$@,$$(@D),$(AR) cr $$@ $$^)
+LIBS_$(2) += build/lib/$(2)/$(3).a
+endef
+
+define lib =
+$(foreach c,$(configs),$(eval $(call archive,AR,$(c),$(1))))
+INCLUDES += -I$(1)/include
+endef
+
+$(foreach l,$(libs),$(eval $(call lib,$(l))))
+
+#
+# Build system functions to generate build rules for all subdirs
+#
+
+sub_makes := $(foreach dir,$(subdirs),$(wildcard ${dir}/*/rules.mk))
+$(foreach makefile,$(sub_makes),$(eval include $(makefile)))
+sub_dirs := $(foreach m,$(sub_makes),$(m:/rules.mk=))
+module_name = $(lastword $(subst /, ,$(1)))
+module_objs = $(addprefix build/obj/$(3)/,$(addprefix $(2)/,$($(1)_objs)))
+config_arch = $(word 1,$(subst :, ,$(1)))
+config_env = $(word 2,$(subst :, ,$(1)))
+
+define rule =
+build/bin/$(3)/$(4)/$(1): \
+build/obj/$(3)/env/$(4)/crt.o $(2) $$(LIBS_$(3))
+	$$(call cmd,LD.$(3) $$@,$$(@D),$(CC_$(3)) $(CFLAGS_$(3)) $$(CFLAGS) \
+	$$(LDFLAGS_$(3)) $$(LDFLAGS) -T env/$(4)/default.lds $$^ -o $$@)
+endef
+
+define module =
+program_names += $(foreach cfg,$(targets),build/bin/$(call \
+  config_arch,$(cfg))/$(call config_env,$(cfg))/$(1))
+
+$(foreach cfg,$(targets),$(eval $(call rule,$(1),$(call \
+  module_objs,$(1),$(2),$(call config_arch,$(cfg))),$(call \
+  config_arch,$(cfg)),$(call config_env,$(cfg)))))
+endef
+
+$(foreach d,$(sub_dirs),$(eval $(call module,$(call module_name,$(d)),$(d))))
+
+all_programs: $(program_names)

README.md

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+# Bare metal RISC-V hello world in C
+
+This is a copy of the [riscv-probe](https://github.com) repository stripped
+from everything but the 64-bit hello world program for the `sifive_u` QEMU
+machine.
+
+The directory structure is kept. This makes this repository a good starting
+point to dive into riscv-probe. In particular we keep the `crt.s` and `start.c`
+files although they are almost empty. They are used in the original repository
+to initialize libfemto. Here libfemto just provides a `puts` function.
+
+Having such a stripped down hello world program written in C was necessary for
+me to learn how to create a [bare metal RISC-V assembly hello
+world](https://github.com) version because I couldn't find a complete example
+program in assembly only that was writing to the UART.
+
+
+## Building
+
+Assuming the default prefix for the GNU toolchain is correct, and the toolchain
+is in the `$PATH`, running `make` produce our `hello` program. Refer to the
+original repository otherwise.
+
+```
+$ make
+AS.rv64imac build/obj/rv64imac/env/qemu-sifive_u/crt.o
+CC.rv64imac build/obj/rv64imac/examples/hello/hello.o
+CC.rv64imac build/obj/rv64imac/libfemto/arch/riscv/start.o
+CC.rv64imac build/obj/rv64imac/libfemto/std/putchar.o
+AR.rv64imac build/lib/rv64imac/libfemto.a
+LD.rv64imac build/bin/rv64imac/qemu-sifive_u/hello
+```
+
+The result is a 64-bit RISC-V binary.
+
+```
+$ file build/bin/rv64imac/qemu-sifive_u/hello
+build/bin/rv64imac/qemu-sifive_u/hello: ELF 64-bit LSB executable, UCB RISC-V,
+version 1 (SYSV), statically linked, not stripped
+```
+
+
+## Running
+
+Run it with:
+
+```
+$ qemu-system-riscv64 -nographic -machine sifive_u \
+  -kernel build/bin/rv64imac/qemu-sifive_u/hello 
+Hello.
+QEMU: Terminated
+```
+
+Note: the program enters an infinite loop after producing the `Hello.` text.
+Type `ctrl-a x` to stop QEMU.
+
+
+## Assembly
+
+To dissamble the program:
+
+```
+$ riscv64-unknown-elf-objdump -d build/bin/rv64imac/qemu-sifive_u/hello
+build/bin/rv64imac/qemu-sifive_u/hello:     file format elf64-littleriscv
+
+Disassembly of section .text:
+
+0000000080000000 <_start>:
+    80000000:	0040006f          j	80000004 <main>
+
+0000000080000004 <main>:
+    80000004:	1141              addi	sp,sp,-16
+    80000006:	e022              sd	s0,0(sp)
+    80000008:	e406              sd	ra,8(sp)
+    8000000a:	00000417          auipc	s0,0x0
+    8000000e:	02e40413          addi	s0,s0,46 # 80000038 <putchar+0x16>
+    80000012:	00044503          lbu	a0,0(s0)
+    80000016:	e111              bnez	a0,8000001a <main+0x16>
+    80000018:	a001              j	80000018 <main+0x14>
+    8000001a:	0405              addi	s0,s0,1
+    8000001c:	006000ef          jal	ra,80000022 <putchar>
+    80000020:	bfcd              j	80000012 <main+0xe>
+
+0000000080000022 <putchar>:
+    80000022:	100137b7          lui	a5,0x10013
+    80000026:	4398              lw	a4,0(a5)
+    80000028:	02071693          slli	a3,a4,0x20
+    8000002c:	fe06cde3          bltz	a3,80000026 <putchar+0x4>
+    80000030:	0ff57513          andi	a0,a0,255
+    80000034:	c388              sw	a0,0(a5)
+    80000036:	8082              ret
+```
+
+I used the above listing to guide modifications to the `hello.S` program
+provided in rv8 (I think) to finally create a pure assembly hello world.

env/qemu-sifive_u/crt.s

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+# See LICENSE for license details.
+
+.section .text.init,"ax",@progbits
+.globl _start
+
+_start:
+    j       main

env/qemu-sifive_u/default.lds

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+OUTPUT_ARCH( "riscv" )
+
+ENTRY( _start )
+
+MEMORY
+{
+  ram   (wxa!ri) : ORIGIN = 0x80000000, LENGTH = 128M
+}
+
+PHDRS
+{
+  text PT_LOAD;
+  data PT_LOAD;
+  bss PT_LOAD;
+}
+
+SECTIONS
+{
+  .text : {
+    PROVIDE(_text_start = .);
+    *(.text.init) *(.text .text.*)
+    PROVIDE(_text_end = .);
+  } >ram AT>ram :text
+
+  .rodata : {
+    PROVIDE(_rodata_start = .);
+    *(.rodata .rodata.*)
+    PROVIDE(_rodata_end = .);
+  } >ram AT>ram :text
+
+  .data : {
+    . = ALIGN(4096);
+    PROVIDE(_data_start = .);
+    *(.sdata .sdata.*) *(.data .data.*)
+    PROVIDE(_data_end = .);
+  } >ram AT>ram :data
+
+  .bss :{
+    PROVIDE(_bss_start = .);
+    *(.sbss .sbss.*) *(.bss .bss.*)
+    PROVIDE(_bss_end = .);
+  } >ram AT>ram :bss
+
+  PROVIDE(_memory_start = ORIGIN(ram));
+  PROVIDE(_memory_end = ORIGIN(ram) + LENGTH(ram));
+}

examples/hello/hello.c

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+#include <stdio.h>
+
+void main()
+{
+    const char *s = "Hello.\n";
+    while (*s) putchar(*s++);
+    while(1);
+}

examples/hello/rules.mk

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+hello_objs = hello.o

libfemto/arch/riscv/start.c

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+// See LICENSE for license details.
+
+void main();
+
+void libfemto_start_main()
+{
+	main();
+}

libfemto/include/stdio.h

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+#pragma once
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+int putchar(int);
+
+#ifdef __cplusplus
+}
+#endif

libfemto/std/putchar.c

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+// See LICENSE for license details.
+
+#include <stdio.h>
+
+enum {
+    /* UART Registers */
+    UART_REG_TXFIFO = 0,
+};
+
+static volatile int *uart = (int *)(void *)0x10013000;
+
+int putchar(int ch)
+{
+    while (uart[UART_REG_TXFIFO] < 0);
+    return uart[UART_REG_TXFIFO] = ch & 0xff;
+}