CAL Example: Hello World

1Learning Outcomes¶

• Identify which step of Compile, Assemble, and Link translates a specific assembly instruction to machine code.

• See an end-to-end example of translation of a hello.c program into an a.out executable.

• Identify assembly directives.

Let’s describe program translation (Compile, Assemble, and Link) with a hello_world example.

2Compiler: hello.c $\rightarrow$ hello.s¶

hello.c:

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#include <stdio.h>
int main() {
    printf("Hello, %s\n", "world");
    return 0;
}

hello.s:

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.text
    .align 2
    .global main
main:
    addi sp sp -4
    sw   ra 0(sp)
    la   a0 str1
    la   a1 str2
    call printf
    lw   ra 0(sp)
    addi sp sp 4
    li   a0 0
    ret
.section .rodata
    .balign 4
str1:
    .string "Hello, %s!\n"
str2:
    .string "world"

Note the many pseudoinstructions: la, call, li, ret, etc.

3Assembler: hello.s $\rightarrow$ hello.o¶

The assembler output is an object file in binary:

     ... ff010113 00112623 00000537
00050513 000005b7 00058593 000080e7
00c12083 01010113 00000513 00008067 ...

3.1Text segment¶

This is not very readable, so we describe some components in more detail. Below is the text segment translating the main portion of hello.o. Refer to hello.s as needed.

00000000 <main>:
0:  ff010113 addi sp sp -16
4:  00112623 sw   ra 12(sp)
8:  00000537 lui  a0 0x0
c:  00050513 addi a0 a0 0
10: 000005b7 lui  a1 0x0
14: 00058593 addi a1 a1 0
18: 000080e7 jalr ra 0
1c: 00c12083 lw   ra 12(sp)
20: 01010113 addi sp sp 16
24: 00000513 addi a0 a0 0
28: 00008067 jalr ra

Pseudoinstructions are replaced where possible.

• la^[1] is replaced with lui and addi

• call is replaced with jalr

The machine code includes address placeholders of zero (e.g., lui a1 0x0 is machine code 000005b7), denoting unresolved references for the linker to resolve.

3.2Symbol Table¶

The below would be in binary, but we illustrate it as a table:

3.3Relocation Table¶

The below would be in binary, but we illustrate it as a table:

4Linker: hello.s $\rightarrow$ a.out¶

A portion of the a.out executable is shown:

000101b0 <main>:
  101b0: ff010113 addi sp sp -16
  101b4: 00112623 sw   ra 12(sp)
  101b8: 00021537 lui  a0 0x21
  101bc: a1050513 addi a0 a0 -1520 # 20a10 <str1>
  101c0: 000215b7 lui  a1 0x21
  101c4: a1c58593 addi a1 a1 -1508 # 20a1c <str2>
  101c8: 288000ef jal  ra 10450    # <printf>
  101cc: 00c12083 lw   ra 12(sp)
  101d0: 01010113 addi sp sp,16
  101d4: 00000513 addi a0 0,0
  101d8: 00008067 jalr ra

• Squash all .o files

• Update in symbol table

• For each entry in the relocation table:

  • Replace placeholders with the actual address

  • Update machine code. Note that the lui upper immediate is incremented by 1; refer to an earlier section about U-Type instruction formats and the li pseudoinstruction.