diff --git a/flake.nix b/flake.nix
index 95becdc..7359d40 100644
--- a/flake.nix
+++ b/flake.nix
@@ -6,6 +6,9 @@
     system = "x86_64-linux";
     pkgs = nixpkgs.legacyPackages.${system};
   in {
-    packages.${system}.default = pkgs.callPackage ./package.nix {};
+    packages.${system} = {
+      default = pkgs.callPackage ./package.nix {};
+      test = pkgs.callPackage ./test.nix {};
+    };
   };
 }
diff --git a/package.nix b/package.nix
index fcdacf6..9969c3e 100644
--- a/package.nix
+++ b/package.nix
@@ -15,8 +15,8 @@ stdenv.mkDerivation {
   configurePhase = ''
   '';
   buildPhase = ''
-    nasm -f elf main.asm
-    ld -m elf_i386 main.o -o main
+    nasm -f elf main_start.asm
+    ld -m elf_i386 main_start.o -o main
   '';
   installPhase = ''
     mkdir -p $out/bin
diff --git a/src/lib.asm b/src/lib.asm
index d464f02..41061b2 100644
--- a/src/lib.asm
+++ b/src/lib.asm
@@ -7,16 +7,16 @@ slen:
   push ebx
   mov ebx, eax
 
-slen_nextchar:
-  cmp byte [eax], 0
-  jz slen_finished
-  inc eax
-  jmp slen_nextchar
+  .loop:
+    cmp byte [eax], 0
+    jz .fin
+    inc eax
+    jmp .loop
 
-slen_finished:
-  sub eax, ebx
-  pop ebx
-  ret
+  .fin:
+    sub eax, ebx
+    pop ebx
+    ret
 
 ; *********************
 ; clear_bss : BSS -> IO
@@ -25,13 +25,13 @@ slen_finished:
 
 clear_bss:
   cmp byte [eax], 0
-  jz clear_bss_finished
+  jz .fin
   mov [eax], byte 0
   inc eax
   jmp clear_bss
 
-clear_bss_finished:
-  ret
+  .fin:
+    ret
 
 ; *******************************
 ; scmp : String -> String -> Bool
@@ -41,25 +41,50 @@ clear_bss_finished:
 scmp:
   push ecx
 
-scmp_nextchar:
-  mov cl, byte [ebx]
-  cmp byte [eax], cl
-  jnz scmp_bad
-  cmp byte [eax], 0
-  jz scmp_good
-  inc eax
-  inc ebx
-  jmp scmp_nextchar
+  .loop:
+    mov cl, byte [ebx]
+    cmp byte [eax], cl
+    jnz .bad
+    cmp byte [eax], 0
+    jz .good
+    inc eax
+    inc ebx
+    jmp .loop
 
-scmp_good:
-  mov eax, 0
-  jmp scmp_finished
+  .good:
+    mov eax, 0
+    jmp .fin
 
-scmp_bad:
-  mov eax, 1
+  .bad:
+    mov eax, 1
 
-scmp_finished:
+  .fin:
+    pop ecx
+    ret
+
+; ************************
+; sdecprint : String -> IO
+; prints string 1 shorter
+; ************************
+
+sdecprint:
+  push edx
+  push ecx
+  push ebx
+  push eax
+  call slen
+  dec eax
+
+  mov edx, eax
+  pop eax
+  mov ecx, eax
+  mov ebx, 1
+  mov eax, 4
+  int 80h
+
+  pop ebx
   pop ecx
+  pop edx
   ret
 
 ; *********************
@@ -104,6 +129,45 @@ sprintln:
   pop eax ; initial
   ret
 
+; *******************
+; bprint : Byte -> IO
+; prints 1 byte
+; *******************
+
+bprint:
+  push edx
+  push ecx
+  push ebx
+  push eax
+
+  mov edx, 1
+  pop eax
+  mov ecx, eax
+  mov ebx, 1
+  mov eax, 4
+  int 80h
+
+  pop ebx
+  pop ecx
+  pop edx
+  ret
+
+; *********************
+; bprintln : Byte -> IO
+; prints 1 byte + 0Ah
+; *********************
+
+bprintln:
+  call bprint
+  push eax
+  mov eax, 0Ah
+  push eax
+  mov eax, esp
+  call bprint
+  pop eax
+  pop eax
+  ret
+
 ; ************
 ; exit : IO
 ; exit program
diff --git a/src/main.asm b/src/main.asm
index c947b6d..de23e10 100644
--- a/src/main.asm
+++ b/src/main.asm
@@ -1,9 +1,18 @@
 %include 'lib.asm'
 
 SECTION .data
-welcome_message db 'Welcome to this test program! It is experimental and very subject to change.', 0h
-prompt          db 'Enter a string to print: ', 0h
-exit_str        db 'exit', 0Ah, 0h
+s:
+  .welcome_message db 'Welcome to this test program! It is experimental and very subject to change.', 0h
+  .prompt          db 'repl>', 0h
+  .error           db '[ERROR] ', 0h
+  .command         db 'command ', 0h
+  .not_found       db ' not found', 0h
+
+fn:
+  .exit_name      db 'exit', 0Ah, 0h
+
+  .help_name      db 'help', 0Ah, 0h
+  .help_content   db 'this program has just 2 commands right now: help and exit', 0Ah, ' help - prints this message', 0Ah, ' exit - quits the program', 0Ah, 'Have fun!', 0h
 
 SECTION .bss
 ; resb ; 0001 byte
@@ -14,48 +23,65 @@ SECTION .bss
 sinput: resb 255 ; 255 bytes
 
 SECTION .text
-global _start
-
-_start:
+start:
   pop ecx ; number of arguments
 
   pop eax ; dispose of first value, the exe
   dec ecx
 
-  mov eax, welcome_message
+  mov eax, s.welcome_message
+  call sprintln
+
+  .arg_loop:
+    cmp ecx, 0h
+    jz .repl
+    pop eax
+    call sprintln
+    dec ecx
+    jmp .arg_loop
+
+  .repl:
+    mov eax, s.prompt
+    call sprint
+
+    mov eax, sinput
+    call clear_bss
+
+    mov edx, 255    ; max bytes to read
+    mov ecx, sinput ; buffer
+    mov ebx, 0      ; (STDIN) where to read from
+    mov eax, 3      ; (SYS_READ)
+    int 80h
+
+    mov eax, sinput
+    mov ebx, fn.exit_name
+    call scmp
+    cmp byte eax, 0
+    jz exit
+
+    mov eax, sinput
+    mov ebx, fn.help_name
+    call scmp
+    cmp byte eax, 0
+    jz fn_help
+
+    mov eax, s.error
+    call sprint
+    mov eax, s.command
+    call sprint
+    mov eax, sinput
+    call sdecprint
+    mov eax, s.not_found
+    call sprintln
+
+    jmp .repl
+
+fn_help:
+  push eax
+
+  mov eax, fn.help_content
   call sprintln
 
-next_arg:
-  cmp ecx, 0h
-  jz prompt_loop
   pop eax
-  call sprintln
-  dec ecx
-  jmp next_arg
 
-prompt_loop:
-  mov eax, prompt
-  call sprint
-
-  mov eax, sinput
-  call clear_bss
-
-  mov edx, 255    ; max bytes to read
-  mov ecx, sinput ; buffer
-  mov ebx, 0      ; (STDIN) where to read from
-  mov eax, 3      ; (SYS_READ)
-  int 80h
-
-  mov eax, sinput
-  mov ebx, exit_str
-  call scmp
-  cmp byte eax, 0
-  jz done
-
-  mov eax, sinput
-  call sprint
-
-  jmp prompt_loop
-
-done:
-  call exit
+  jmp start.repl
diff --git a/src/main_start.asm b/src/main_start.asm
new file mode 100644
index 0000000..7d284a7
--- /dev/null
+++ b/src/main_start.asm
@@ -0,0 +1,7 @@
+%include 'main.asm'
+
+SECTION .text
+global _start
+
+_start:
+  jmp start
diff --git a/src/test_start.asm b/src/test_start.asm
new file mode 100644
index 0000000..0a67a01
--- /dev/null
+++ b/src/test_start.asm
@@ -0,0 +1,242 @@
+%include 'lib.asm'
+
+SECTION .data
+ts:
+  .test_welcome db "Welcome to the test suite!", 0h
+
+  .running_test db "running test ", 0h
+  .ellipsis     db "... ", 0h
+  .error        db "[ERROR] ", 0h
+  .failed       db " failed", 0h
+  .failed_code  db " failed with exit code ", 0h
+  .all_good     db "all tests passed", 0h
+  .divider      db "--------------------------------", 0h
+
+slen_zero_length:
+  .name   db "slen_zero_length", 0h
+  .should db "slen should be 0", 0h
+  .result db "slen result is ", 0h
+
+scmp_a_zero_length:
+  .name   db "scmp_a_zero_length", 0h
+  .should db "scmp should be 1", 0h
+  .result db "scmp result is ", 0h
+
+scmp_b_zero_length:
+  .name   db "scmp_b_zero_length", 0h
+  .should db "scmp should be 1", 0h
+  .result db "scmp result is ", 0h
+
+scmp_both_zero_length:
+  .name   db "scmp_both_zero_length", 0h
+  .should db "scmp should be 0", 0h
+  .result db "scmp result is ", 0h
+
+scmp_both_short_s:
+  .name   db "scmp_both_short_s", 0h
+  .should db "scmp should be 0", 0h
+  .result db "scmp result is ", 0h
+
+cases:
+  .zero_length db 0h
+  .short_s     db "1 short Test String!", 0h
+
+numerals db "0123456789", 0h
+
+SECTION .text
+global _start
+
+_start:
+  mov eax, ts.test_welcome
+  call sprintln
+
+  mov eax, ts.divider
+  call sprintln
+  mov eax, slen_zero_length.name
+  call sprintln
+  call slen_zero_length_test
+
+  mov eax, ts.divider
+  call sprintln
+  mov eax, scmp_a_zero_length.name
+  call sprintln
+  call scmp_a_zero_length_test
+
+  mov eax, ts.divider
+  call sprintln
+  mov eax, scmp_b_zero_length.name
+  call sprintln
+  call scmp_b_zero_length_test
+
+  mov eax, ts.divider
+  call sprintln
+  mov eax, scmp_both_zero_length.name
+  call sprintln
+  call scmp_both_zero_length_test
+
+  mov eax, ts.divider
+  call sprintln
+  mov eax, scmp_both_short_s.name
+  call sprintln
+  call scmp_both_short_s_test
+
+  mov eax, ts.divider
+  call sprintln
+  mov eax, ts.all_good
+  call sprintln
+
+  call exit
+
+slen_zero_length_test:
+  push eax
+
+  mov eax, cases.zero_length
+  call slen
+  push eax
+  mov eax, slen_zero_length.should
+  call sprintln
+  mov eax, slen_zero_length.result
+  call sprint
+  pop eax
+  cmp eax, 0
+  jnz slen_zero_length_test.failed
+  add eax, numerals
+  call bprintln
+
+  pop eax
+  ret
+
+  .failed:
+    mov ebx, slen_zero_length.name
+    call failed_with_code
+
+scmp_a_zero_length_test:
+  push ebx
+  push eax
+
+  mov eax, cases.zero_length
+  mov ebx, cases.short_s
+  call scmp
+  push eax
+  mov eax, scmp_a_zero_length.should
+  call sprintln
+  mov eax, scmp_a_zero_length.result
+  call sprint
+  pop eax
+  cmp eax, 1
+  jnz scmp_a_zero_length_test.failed
+  add eax, numerals
+  call bprintln
+
+  pop eax
+  pop ebx
+  ret
+
+  .failed:
+    mov ebx, scmp_a_zero_length.name
+    call failed_with_code
+
+scmp_b_zero_length_test:
+  push ebx
+  push eax
+
+  mov eax, cases.short_s
+  mov ebx, cases.zero_length
+  call scmp
+  push eax
+  mov eax, scmp_b_zero_length.should
+  call sprintln
+  mov eax, scmp_b_zero_length.result
+  call sprint
+  pop eax
+  cmp eax, 1
+  jnz scmp_b_zero_length_test.failed
+  add eax, numerals
+  call bprintln
+
+  pop eax
+  pop ebx
+  ret
+
+  .failed:
+    mov ebx, scmp_b_zero_length.name
+    call failed_with_code
+
+scmp_both_zero_length_test:
+  push ebx
+  push eax
+
+  mov eax, cases.zero_length
+  mov ebx, eax
+  call scmp
+  push eax
+  mov eax, scmp_both_zero_length.should
+  call sprintln
+  mov eax, scmp_both_zero_length.result
+  call sprint
+  pop eax
+  cmp eax, 0
+  jnz scmp_both_zero_length_test.failed
+  add eax, numerals
+  call bprintln
+
+  pop eax
+  pop ebx
+  ret
+
+  .failed:
+    mov ebx, scmp_both_zero_length.name
+    call failed_with_code
+
+scmp_both_short_s_test:
+  push ebx
+  push eax
+
+  mov eax, cases.short_s
+  mov ebx, eax
+  call scmp
+  push eax
+  mov eax, scmp_both_short_s.should
+  call sprintln
+  mov eax, scmp_both_short_s.result
+  call sprint
+  pop eax
+  cmp eax, 0
+  jnz scmp_both_zero_length_test.failed
+  add eax, numerals
+  call bprintln
+
+  pop eax
+  pop ebx
+  ret
+
+  .failed:
+    mov ebx, scmp_both_zero_length.name
+    call failed_with_code
+
+failed:
+  mov eax, ts.ellipsis
+  call sprintln
+  mov eax, ts.error
+  call sprint
+  mov eax, ebx
+  call sprint
+  mov eax, ts.failed
+  call sprint
+  call exit
+
+failed_with_code:
+  push eax
+  
+  mov eax, ts.ellipsis
+  call sprintln
+  mov eax, ts.error
+  call sprint
+  mov eax, ebx
+  call sprint
+  mov eax, ts.failed_code
+  call sprint
+  pop eax
+  add eax, numerals
+  call bprintln
+  call exit
diff --git a/test.nix b/test.nix
new file mode 100644
index 0000000..573ecde
--- /dev/null
+++ b/test.nix
@@ -0,0 +1,30 @@
+{
+  lib,
+  stdenv,
+  nasm,
+  ...
+}:
+stdenv.mkDerivation {
+  name = "asm-test";
+  src = ./src;
+  nativeBuildInputs = [
+    nasm
+  ];
+  buildInputs = [
+  ];
+  configurePhase = ''
+  '';
+  buildPhase = ''
+    nasm -f elf test_start.asm
+    ld -m elf_i386 test_start.o -o test
+  '';
+  installPhase = ''
+    mkdir -p $out/bin
+    cp test $out/bin/test
+  '';
+
+  meta = {
+    license = lib.licenses.wtfpl;
+    mainProgram = "test";
+  };
+}