Small number output in PDP11 machine code
=========================================

; Fixed digits decimal output
; ===========================

; A quick two-digit binary to decimal conversion
; Outputs '00' to '99' for input of 0-99
; On entry, R1=0-99
; Uses      R0 for digit
;
MOVB #ASC"0"-1,R0        ; Start with '0'-1 for tens
.PrDecTens
INC  R0                  ; Increment tens digit
SUB  #10,R1              ; Subtract 10 from R0
BCC  PrDecTens           ; Loop until <0
JSR  PC,output_character ; Output the character in R0

ADD  #ASC"0"+10,R1       ; Convert R1 to units digit
JSR  PC,output_character ; Output the character in R0


; You can also write the digits to a buffer with the following
; On entry, R1=0-99
;           R0=>buffer
;
MOVB #ASC"0"-1,(R0)      ; Start with '0'-1 for tens
.PrDecTens
INCB (R0)                ; Increment tens digit
SUB  #10,R1              ; Subtract 10 from R0
BCC  PrDecTens           ; Loop until <0
INC  R0                  ; Step past the tens digit
ADD  #ASC"0"+10,R1       ; Convert R1 to units digit
MOVB R1,(R0)+            ; Store the units digit


; You can increase the number of digits by adding extra loops
;
MOVB #ASC"0"-1,(R0)      ; Start with '0'-1 for hundreds
.PrDecHundreds
INCB (R0)                ; Increment hundreds digit
SUB  #100,R1             ; Subtract 100 from R0
BCC  PrDecHundreds       ; Loop until <0
INC  R0                  ; Step past hundreds digit
ADD  #100,R1             ; Balance last SUB #100

MOVB #ASC"0"-1,(R0)      ; Start with '0'-1 for tens
.PrDecTens
INCB (R0)                ; Increment tens digit
SUB  #10,R1              ; Subtract 10 from R0
BCC  PrDecTens           ; Loop until <0
INC  R0                  ; Step past the tens digit

ADD  #ASC"0"+10,R1       ; Convert R1 to units digit
MOVB R1,(R0)+            ; Store the units digit


; If doing more than three digits it is more efficient to use a table
; of powers of ten and loop through them