#x86-16 machine code, IBM PC DOS, (削除) 27 (削除ここまで) 23 bytes

Binary:

00000000: b408 cd21 b40e cd10 483c 617d f9b0 0acd ...!....H<a}....
00000010: 10b0 0dcd 10eb e9 .......

Test and build using xxd -r from above.

Unassembled:

 IN_LOOP: 
B4 08 MOV AH, 08H ; DOS read char without echo function
CD 21 INT 21H ; load next char from STDIN
B4 0E MOV AH, 0EH ; BIOS write char teletype output function
 CHR_LOOP: 
CD 10 INT 10H ; write to console 
48 DEC AX ; move to next char 
3C 61 CMP AL, 'a' ; end with an 'a' 
7D F9 JGE CHR_LOOP ; keep looping descending chars 
B0 0A MOV AL, 0AH ; CR char 
CD 10 INT 10H ; write to console 
B0 0D MOV AL, 0DH ; LF char 
CD 10 INT 10H ; write to console 
EB E9 JMP IN_LOOP ; keep looping until ^C/^Break

I/O:

Input via STDIN or interactive input, output to console. Will run until EOF or ^C/^Break.

enter image description here

Iterative approach. Of course, it's 8 bytes just to write a newline character...

x86-16 machine code, IBM PC DOS, (削除) 27 (削除ここまで) 23 bytes

Binary:

00000000: b408 cd21 b40e cd10 483c 617d f9b0 0acd ...!....H<a}....
00000010: 10b0 0dcd 10eb e9 .......

Test and build using xxd -r from above.

Unassembled:

 IN_LOOP: 
B4 08 MOV AH, 08H ; DOS read char without echo function
CD 21 INT 21H ; load next char from STDIN
B4 0E MOV AH, 0EH ; BIOS write char teletype output function
 CHR_LOOP: 
CD 10 INT 10H ; write to console 
48 DEC AX ; move to next char 
3C 61 CMP AL, 'a' ; end with an 'a' 
7D F9 JGE CHR_LOOP ; keep looping descending chars 
B0 0A MOV AL, 0AH ; CR char 
CD 10 INT 10H ; write to console 
B0 0D MOV AL, 0DH ; LF char 
CD 10 INT 10H ; write to console 
EB E9 JMP IN_LOOP ; keep looping until ^C/^Break

I/O:

Input via STDIN or interactive input, output to console. Will run until EOF or ^C/^Break.

enter image description here

Iterative approach. Of course, it's 8 bytes just to write a newline character...

#x86-16 machine code, IBM PC DOS, 27 bytes

00000000: d1ee adb4 0e8a c849 accd 1048 3c61 7df9 .......I...H<a}.
00000010: b00a cd10 b00d cd10 e2ee c3 ...........

D1 EE  SHR SI, 1  ; SI to DOS command line input (80H) 
AD LODSW ; AL = input string length 
B4 0E MOV AH, 0EH ; BIOS write char function
8A C8  MOV CL, AL ; string length into CX 
49  DEC CX ; remove leading char from length count 
  IN_LOOP: 
AC  LODSB  ; load next char 
 CHR_LOOP: 
CD 10 INT 10H ; write to console 
48 DEC AX ; move to next char 
3C 61 CMP AL, 'a' ; end with an 'a' 
7D F9 JGE CHR_LOOP ; keep looping descending chars 
B0 0A MOV AL, 0AH ; CR char 
CD 10 INT 10H ; write to console 
B0 0D MOV AL, 0DH ; LF char 
CD 10 INT 10H ; write to console 
E2 EE LOOP IN_LOOP ; keep looping input string 
C3 RET  ; return to DOS 

Example I/O:

enter image description here

#x86-16 machine code, IBM PC DOS, (削除) 27 (削除ここまで) 23 bytes

00000000: b408 cd21 b40e cd10 483c 617d f9b0 0acd ...!....H<a}....
00000010: 10b0 0dcd 10eb e9 .......

 IN_LOOP: 
B4 08 MOV AH, 08H ; DOS read char without echo function
CD 21 INT 21H ; load next char from STDIN
B4 0E MOV AH, 0EH ; BIOS write char teletype output function
 CHR_LOOP: 
CD 10 INT 10H ; write to console 
48 DEC AX ; move to next char 
3C 61 CMP AL, 'a' ; end with an 'a' 
7D F9 JGE CHR_LOOP ; keep looping descending chars 
B0 0A MOV AL, 0AH ; CR char 
CD 10 INT 10H ; write to console 
B0 0D MOV AL, 0DH ; LF char 
CD 10 INT 10H ; write to console 
EB E9 JMP  IN_LOOP ; keep looping until ^C/^Break

I/O:

Input via STDIN or interactive input, output to console. Will run until EOF or ^C/^Break.

enter image description here

Iterative approach. Of course, it's 8 bytes just to write a newline character...