PROC and ENDP are assembler directives, not instructions: they produce no code of their own. name PROC defines name as a code label and records whether it is NEAR or FAR. MASM uses that in two places: a CALL name becomes a near or far call, and every RET between PROC and ENDP becomes a near or far return. Without NEAR or FAR, the distance follows the .MODEL: NEAR for TINY, SMALL and COMPACT, FAR for MEDIUM, LARGE and HUGE.

MOV     AL, 1
CALL    SHOWD1
MOV     AL, 2
CALL    SHOWD1
MOV     AL, 3
CALL    SHOWD1        ; prints 123
 
SHOWD1  PROC    NEAR  ; print AL as a digit; keeps AX and DX
PUSH    AX
PUSH    DX
ADD     AL, "0"
MOV     DL, AL
MOV     AH, 02h
INT     21h
POP     DX
POP     AX
RET
SHOWD1  ENDP

Pitfalls

PROC is not a wall: if the program runs into it from above, the CPU executes it like any other code, and its RET then pops a random word. Place procedures after the exit, as here, or jump over them. ENDP does not return either; a missing RET falls through into whatever follows. The name on ENDP must match the one on PROC. In MASM 6 a label with one colon inside a procedure is local to it, so two procedures may each have a DONE1:; write DONE1:: to make one visible outside.

MASM extras

MASM 6 accepts more on the PROC line. USES SI DI saves and restores the listed registers, and with a language type, parameters can be named so MASM writes the BP frame for you. This guide writes frames by hand (see Stack frames) so every instruction is visible.

FAR1    PROC    FAR  ; called with CALL FAR PTR; RET becomes RETF
RET
FAR1    ENDP
SUM2    PROC    NEAR C, A1:WORD, B1:WORD
MOV     AX, A1       ; MASM reads [BP+4]
ADD     AX, B1       ; and [BP+6]
RET                  ; MASM adds POP BP before it
SUM2    ENDP