diff --git a/dosdl.asm b/dosdl.asm
index 09b298f..91bac69 100644
--- a/dosdl.asm
+++ b/dosdl.asm
@@ -80,6 +80,11 @@ seed_rng_date:
 	int 0x21
 	mov [year], cx
 	mov [month], dh
+%ifdef DEBUG
+	mov cx, 2022
+	mov dh, 2
+	mov dl, 24
+%endif
 	mov [day], dl
 
 	; addl32 = year * 10000
@@ -211,6 +216,98 @@ read_guess:
 
 	.done:
 
+check_hard_mode:
+	cmp byte [hard_mode], 1
+	jne .done
+
+	; See whether exact hits are reused
+	xor bp, bp
+	mov cx, 5
+	.exact_hits:
+		mov al, [exact_hits + bp]
+		test al, al
+		jz .not_exact_hit
+
+		cmp al, [guess + bp]
+		je .exact_hit_match
+
+		mov dx, bp
+		add dl, '1'
+		mov [exact_hit_error_str.position], dl
+		mov [exact_hit_error_str.letter], al
+		mov si, exact_hit_error_str
+		jmp print_error
+
+		.exact_hit_match:
+		.not_exact_hit:
+		inc bp
+		loop .exact_hits
+
+	; See whether wrongly placed letters are reused
+	xor bp, bp
+	.wrong_places:
+		cmp bp, 5
+		je .done
+
+		mov dl, [wrong_places + bp]
+		test dl, dl
+		jz .not_wrong_place
+
+		; Count how many times letter is used in exact_hits + wrong_places
+		xor bx, bx
+		mov cx, 5
+		mov si, exact_hits
+		.count_exact_hits:
+			lodsb
+			cmp al, dl
+			jne .not_count_exact_hit
+			inc bh
+			.not_count_exact_hit:
+			loop .count_exact_hits
+		mov cx, 5
+		mov si, wrong_places
+		.count_wrong_places:
+			lodsb
+			cmp al, dl
+			jne .not_count_wrong_places
+			inc bh
+			.not_count_wrong_places:
+			loop .count_wrong_places
+
+		; Count how many times letter is used in guess
+		mov cx, 5
+		mov si, guess
+		.count_guess:
+			lodsb
+			cmp al, dl
+			jne .not_count_guess
+			inc bl
+			.not_count_guess:
+			loop .count_guess
+
+		cmp bl, bh
+		jae .used_enough
+
+		mov [wrong_place_error_str.letter], dl
+		mov ax, bp
+		add ax, '1'
+		mov [wrong_place_error_str.position], al
+		mov si, wrong_place_error_str
+		jmp print_error
+
+		.used_enough:
+		.not_wrong_place:
+		inc bp
+		jmp .wrong_places
+
+	.done:
+
+check_ultra_hard_mode:
+	cmp byte [ultra_hard_mode], 1
+	jne .done
+
+	.done:
+
 check_dictionary:
 	; Dictionary is split by initial letter
 	xor bx, bx
@@ -848,7 +945,20 @@ addr32 dd 0
 
 year dw 0
 month dw 0
-day db 0
+day dw 0
+
+exact_hit_error_str:
+	db ' - letter at position '
+	.position db '#'
+	db " must be '"
+	.letter db '#'
+	db "'$"
+wrong_place_error_str:
+	db " - letter '"
+	.letter db '#'
+	db "' from position "
+	.position db '#'
+	db ' must be reused$'
 
 section .rodata
 
@@ -863,7 +973,9 @@ guesses_str db ' guesses.$'
 guess_str db ' guess.$'
 
 help_str:
-	db 'Usage: dosdl', 13, 10
+	db 'Usage: dosdl [/h | /u]', 13, 10
+	db '/h   Enable hard mode.', 13, 10
+	db '/u   Enable ultra hard mode.', 13, 10
 	db 13, 10
 	db 'Hello DOSdl is a word guessing game. You have six tries to guess the correct', 13, 10
 	db 'English word. After a guess the game displays feedback under each letter:', 13, 10
@@ -874,7 +986,12 @@ help_str:
 	db '    above cases.', 13, 10
 	db 13, 10
 	db 'After the feedback, the game shows a list of letters that have not yet been', 13, 10
-	db 'ruled out.$'
+	db 'ruled out.', 13, 10
+	db 13, 10
+	db 'In hard mode all letters marked as being in the correct place must stay fixed', 13, 10
+	db 'and those marked as being in the wrong place must be reused. In ultra hard mode', 13, 10
+	db 'letters marked as being in the wrong place must also be moved and letters not', 13, 10
+	db 'in the word must not be played again.$'
 
 %include "dictionary.inc"
 %include "targets.inc"