diff --git a/src/crt/cttz.src b/src/crt/cttz.src
new file mode 100644
index 000000000..6e938859f
--- /dev/null
+++ b/src/crt/cttz.src
@@ -0,0 +1,128 @@
+	assume	adl=1
+
+	section	.text
+	public	__bcttz
+__bcttz:
+	cp	a, 1
+	require	__cttz_common
+
+	section	.text
+	private	__cttz_common
+	; Input: A=byte, CF=(A==0)
+	; Output: A=cttz(A)
+__cttz_common:
+	adc	a, a
+	add	a, a
+	jr	z, .high2
+	add	a, a
+	add	a, a
+	jr	z, .high4
+	add	a, a
+	add	a, a
+	jr	z, .high6
+	add	a, a
+	add	a, a
+	sbc	a, -1
+	ret	p
+	ld	a, 8
+	ret
+.high6:
+	sbc	a, -3
+	ret
+.high4:
+	sbc	a, -5
+	ret
+.high2:
+	sbc	a, -7
+	ret
+
+	section	.text
+	public	__scttz
+__scttz:
+	xor	a, a
+	or	a, l
+	jr	nz, __cttz_common
+	sub	a, h
+	ccf
+	require	__scttz.hijack
+
+	section	.text
+	private	__scttz.hijack
+__scttz.hijack:
+	call	__cttz_common
+	add	a, 8
+	ret
+
+	section	.text
+	public	__icttz
+__icttz:
+	xor	a, a
+	require	__icttz.hijack
+
+	section	.text
+	private	__icttz.hijack
+__icttz.hijack:
+	or	a, l
+	jr	nz, __cttz_common
+	or	a, h
+	jr	nz, __scttz.hijack
+	dec	sp
+	push	hl
+	inc	sp
+	pop	af
+	call	__bcttz
+	add	a, 16
+	ret
+
+	section	.text
+	public	__lcttz
+__lcttz:
+	add	hl, de
+	xor	a, a
+	sbc	hl, de
+	jr	nz, __icttz.hijack
+	sub	a, e
+	ccf
+	call	__cttz_common
+	add	a, 24
+	ret
+
+	section	.text
+	public	__llcttz
+__llcttz:
+	add	hl, de
+	xor	a, a
+	sbc	hl, de
+	jr	nz, __icttz.hijack
+	ex	de, hl
+	sbc	hl, de
+	jr	nz, __i48cttz.hijack
+	or	a, c
+	jr	nz, .low
+	sub	a, b
+	ccf
+	call	__cttz_common
+	add	a, 56
+	ret
+.low:
+	call	__cttz_common
+	add	a, 48
+	ret
+
+	section	.text
+	public	__i48cttz
+__i48cttz:
+	add	hl, de
+	xor	a, a
+	sbc	hl, de
+	jr	nz, __icttz.hijack
+	ex	de, hl
+	require	__i48cttz.hijack
+
+	section	.text
+	private	__i48cttz.hijack
+__i48cttz.hijack:
+	call	__icttz.hijack
+	ex	de, hl
+	add	a, 24
+	ret
diff --git a/src/libc/ez80_builtin.src b/src/libc/ez80_builtin.src
index 004e724c3..e567b1cc3 100644
--- a/src/libc/ez80_builtin.src
+++ b/src/libc/ez80_builtin.src
@@ -6,18 +6,10 @@
 
 	public	___ez80_ctzc
 ___ez80_ctzc:
-	; unoptimized
 	ld	hl, 3
 	add	hl, sp
-	ld	l, (hl)
-	xor	a, a
-	sub	a, l
-	and	a, l
-	call	__bctlz
-	bit	3, a
-	ret	nz
-	xor	a, 7
-	ret
+	ld	a, (hl)
+	jp	__bcttz
 
 ;-------------------------------------------------------------------------------
 
@@ -25,25 +17,14 @@ ___ez80_ctzc:
 
 	public	___ez80_ctzi48
 ___ez80_ctzi48:
-	; unoptimized
-	ld	hl, 3
+	ld	hl, 6
 	add	hl, sp
-	ld	bc, (hl)
-	inc	hl
-	inc	hl
-	inc	hl
-	ld	iy, (hl)
-	sbc	hl, hl
-	add	hl, bc
-	lea	de, iy
-	call	__i48neg
-	call	__i48and
-	call	__i48ctlz
-	cpl
-	add	a, 48
-	ret	p
-	ld	a, 48
-	ret
+	ld	de, (hl)
+	dec	hl
+	dec	hl
+	dec	hl
+	ld	hl, (hl)
+	jp	__i48cttz
 
 ;-------------------------------------------------------------------------------
 
@@ -51,16 +32,12 @@ ___ez80_ctzi48:
 
 	public	___ez80_ffsc
 ___ez80_ffsc:
-	; unoptimized
 	ld	hl, 3
 	add	hl, sp
-	ld	l, (hl)
-	xor	a, a
-	sub	a, l
-	and	a, l
-	call	__bctlz
-	cpl
-	add	a, 9
+	ld	a, (hl)
+	or	a, a
+	rla
+	jp	nz, __bcttz
 	ret
 
 ;-------------------------------------------------------------------------------
@@ -69,24 +46,13 @@ ___ez80_ffsc:
 
 	public	___ez80_ffss
 ___ez80_ffss:
-	; unoptimized
 	ld	hl, 3
 	add	hl, sp
 	ld	hl, (hl)
-	; HL & -HL
-	ld	b, h
-	ld	c, l
-	sbc	hl, hl
-	sbc	hl, bc
 	ld	a, h
-	and	a, b
-	ld	h, a
-	ld	a, l
-	and	a, c
-	ld	l, a
-	call	__sctlz
-	cpl
-	add	a, 17
+	or	a, l
+	add	hl, hl
+	jp	nz, __scttz
 	ret
 
 ;-------------------------------------------------------------------------------
@@ -95,22 +61,18 @@ ___ez80_ffss:
 
 	public	___ez80_ffsi48
 ___ez80_ffsi48:
-	; unoptimized
-	ld	hl, 3
+	ld	hl, 6
 	add	hl, sp
-	ld	bc, (hl)
-	inc	hl
-	inc	hl
-	inc	hl
-	ld	iy, (hl)
-	sbc	hl, hl
-	add	hl, bc
-	lea	de, iy
-	call	__i48neg
-	call	__i48and
-	call	__i48ctlz
-	cpl
-	add	a, 49
+	ld	de, (hl)
+	dec	hl
+	dec	hl
+	dec	hl
+	ld	hl, (hl)
+	call	__i48cttz
+	inc	a
+	cp	a, 49
+	ret	nz
+	xor	a, a
 	ret
 
 ;-------------------------------------------------------------------------------
@@ -406,13 +368,15 @@ ___ez80_rotateright48:
 
 	extern	__snot
 	extern	__i48not
-	extern	__i48and
-	extern	__i48neg
 
 	extern	__bctlz
 	extern	__sctlz
 	extern	__i48ctlz
 
+	extern	__bcttz
+	extern	__scttz
+	extern	__i48cttz
+
 	extern	__bpopcnt
 	extern	__spopcnt
 	extern	__i48popcnt
diff --git a/test/standalone/ez80_builtin/src/main.c b/test/standalone/ez80_builtin/src/main.c
index 30e97e53f..4ef4312ca 100644
--- a/test/standalone/ez80_builtin/src/main.c
+++ b/test/standalone/ez80_builtin/src/main.c
@@ -121,7 +121,7 @@ static int test_ctzc(void) {
     unsigned char input;
     for (int i = 0; i < 256; i++) {
         input = (unsigned char)i;
-        truth = (input == 0) ? 8 : __builtin_ctz((unsigned int)input);
+        truth = (input == 0) ? 8 : __builtin_ctz((unsigned int)input | INT_MIN);
         guess = __ez80_ctzc(input);
         CMP("%d", input, truth, guess);
     }
@@ -271,7 +271,7 @@ static int test_ctzi48(void) {
     CMP("%012llX", (uint64_t)UINT48_MAX, 0, __ez80_ctzi48(UINT48_MAX));
     for (int i = 0; i < RANDOM_TEST_COUNT; i++) {
         input = rand48();
-        truth = (input == 0) ? 48 : __builtin_ctzll((unsigned long long)input);
+        truth = (input == 0) ? 48 : __builtin_ctzll((unsigned long long)input | INT64_MIN);
         guess = __ez80_ctzi48(input);
         CMP("%012llX", (uint64_t)input, truth, guess);
     }