Add comments and loops for hello assembly program

It would be one of the student exercises.

Author: jserv

tests/asm-hello/hello.S

@@ -1,24 +1,40 @@
+# RISC-V assembly program to print "Hello World!" to stdout.
+
 .org 0
+# Provide program starting address to linker
 .global _start
 
-.data
-
-str: .ascii "Hello World!\n"
-     .set str_size, .-str
-
 /* newlib system calls */
 .set SYSEXIT,  93
 .set SYSWRITE, 64
 
-.text
+.data
+str: .ascii "Hello World!\n"
+     .set str_size, .-str
 
+.text
 _start:
-    li a7, SYSWRITE
-    li a0, 1
-    la a1, str
-    li a2, str_size
-    ecall
-
-    li a7, SYSEXIT
-    add a0, x0, 0
-    ecall
+    li t0, 0
+    li t1, 5
+
+    # dummy test for jal instruction
+.L1:
+    jal x0, .L2
+.L2:
+    nop
+
+loop:
+    beq t0, t1, end
+
+    li a7, 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 a7, SYSEXIT      # "exit" syscall
+    add a0, x0, 0       # Use 0 return code
+    ecall               # invoke syscall to terminate the program

tests/asm-hello/hello.ld

@@ -1,4 +1,4 @@
-OUTPUT_ARCH( "riscv" )
+OUTPUT_ARCH("riscv")
 ENTRY(_start)
 
 SECTIONS