;;#$15 - Max Right Walking Speed
;;#$31 - Max Right Running Speed
;;#$EB - Max Left Walking Speed
;;#$CF - Max Right Running Speed

;;$7E0073 - Is Mario Ducking flag - #$04 = yes ; Is not flagged as ducking while sliding.

lorom

org $00D600
autoclean JML duck_check

freecode
duck_check:

LDA $15		;; / Original Code
AND #$04	;; | If Duck isn't pressed, stand up
BEQ $D60B	;; \

;;If Mario is already Ducked resume
LDA.W $7E0073
CMP #$04
BEQ resume

LDA.W $7E007B	;; RAM address of Marios X Speed
CMP #$EB	;; / If X Speed is greater then max left walking speed
BCS force_stand ;; \ bigger number is slower so force stand

CMP #$15	;; / if X Speed is less then
BCC force_stand ;; \ max right walking speed

JML resume

force_stand: ;; Enters here when speed is 14
JML $00D60B

resume:
LDA $15		;; We jump back to an STA so need to restore A
AND.B #$04	;; back to exactly how it was
JML $00D606	;; Jump back to regular execution