# print a string in reverse, one-char-at-a-time, using a loop # # code uses registers a0, a7, t0, t1, t2 # # Note: string contains only ASCII characters. UTF-8 has many more # multi-byte characters. .data string: .asciz "Wow, does it work?" .text main: # ---------- step 1. set string address and string char (byte) index la t0, string # address of the first char (byte) of a string li t1, 0 # initialize string char index # ---------- step 2. loop looking for the end-of-the-string (null char) locate_end_of_string: # step 2.1 load string's (ASCII) char into a0 add t2, t1, t0 # set t2 to address of string char (t0 + t1) lbu a0, 0(t2) # load char (byte) unsigned # step 2.2 end of string? (a0 == null char) beq a0, zero, found_end_of_string # If string char is null, exit the loop # step 2.3 increment string char (byte) index # jump to locate-end-of-string loop addi t1, t1, 1 # increment string char index j locate_end_of_string # loop found_end_of_string: addi t1, t1, -1 # decrement string char index # to last char in string # ---------- step 3. print string in reverse start_print_loop: # step 3.1 end of string? (a0 == 0) (a0 == null char) blt t1, zero, end_print_loop # if string char index < zero, # stop printing # step 3.2 load string's (ASCII) char into a0 add t2, t1, t0 # set t2 to address of string char # (t0 + t1) lbu a0, 0(t2) # load char (byte) unsigned # step 3.3 print string char li a7, 11 # service code 11 = Print character ecall # step 3.4 decrement string char (byte) index # jump to beginning of print loop addi t1, t1, -1 # decrement string char index j start_print_loop # loop end_print_loop: # ---------- step 4. exit program li a0, 0 # returned value li a7, 93 # exit program ecall # Trigger exit system call