Add some sample files

.gitignore

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+*.o
+*.elf

Makefile

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+AS=riscv64-unknown-elf-as
+LD=riscv64-unknown-elf-ld
+
+all: main.x
+
+hello.elf: hello.o
+	$(LD) -m elf32lriscv $^ -o $@
+
+main.x: main.o exit.o
+	$(LD) -m elf32lriscv $^ -o main.x
+
+%.o : %.s
+	$(AS) -mabi=ilp32e -march=rv32ec $< -o $@

README.md

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+Calculator
+==========
+
+Aims to be a calculator implemented in 32-bit RISC-V assembly, initially targeting the
+inexpensive [CH32V003 RV32EC microcontroller][CH32V003]: 48Mhz 2KB SRAM, 16KB Flash.
+
+Notes
+-----
+
+### Disassemble
+
+    riscv64-unknown-elf-objdump -D main.o
+
+Resources
+---------
+
+* https://riscv-programming.org/book/riscv-book.html
+* [CH32V003 Datasheet](https://www.wch-ic.com/downloads/CH32V003RM_PDF.html)
+* [QingKeV2 Processor Manual](https://www.wch-ic.com/downloads/QingKeV2_Processor_Manual_PDF.html) — core used in the CH32V003
+
+
+[CH32V003]: https://www.wch-ic.com/products/CH32V003.html

exit.s

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+.globl exit
+exit:
+  li a0, 0
+  li a1, 93
+  ecall

hello.s

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+# RISC-V assembly program to print "Hello World!" to stdout.
+
+.org 0
+# Provide program starting address to linker
+.global _start
+
+/* newlib system calls */
+.set SYSEXIT,  93
+.set SYSWRITE, 64
+
+.section .rodata
+str: .ascii "Hello World!\n"
+     .set str_size, .-str
+
+.text
+_start:
+    li t0, 0
+    li t1, 5
+
+    # dummy test for jal instruction
+.L1:
+    jal x0, .L2
+.L2:
+    nop
+
+loop:
+    beq t0, t1, end
+
+    li t0, SYSWRITE     # "write" syscall
+    li a0, 1            # 1 = standard output (stdout)
+    la a1, str          # load address of hello string
+    li a2, str_size     # length of hello string
+    ecall               # invoke syscall to print the string
+    addi t0, t0, 1
+    j loop
+
+end:
+    li t0, SYSEXIT      # "exit" syscall
+    add a0, x0, 0       # Use 0 return code
+    ecall               # invoke syscall to terminate the program

main.s

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+.globl _start
+_start:
+  li a0, 10
+  li a1, 20
+  jal exit