From 636f688ac516d2d2bed4e0e8836a7f0720bd9958 Mon Sep 17 00:00:00 2001
From: Mark Wooding <mdw@distorted.org.uk>
Date: Mon, 19 Oct 2020 05:35:37 +0100
Subject: [PATCH] xchg.S: Another couple of (easy) exercises.

---
 xchg.S | 67 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 59 insertions(+), 8 deletions(-)

diff --git a/xchg.S b/xchg.S
index c99e0a6..40c5ba9 100644
--- a/xchg.S
+++ b/xchg.S
@@ -2750,50 +2750,101 @@ endproc
 
 proc	x26
 
+	// a bad way to rotate a right by 7 places
+
 #if defined(__x86_64__)
 
-	notimpl
+	mov	rbx, rax
+	ror	rbx, 7			// better
+
+	mov	rdx, rax		// d' = a
+	shr	rax, 7			// a' = a >> 7
+	shl	rdx, 0x39		// d' = a << 57
+	or	rax, rdx		// a' = a >>> 7
 
 #elif defined(__i386__)
 
-	notimpl
+	mov	ebx, eax
+	ror	ebx, 7			// better
+
+	mov	edx, eax		// d' = a
+	shr	eax, 7			// a' = a >> 7
+	shl	edx, 0x39		// d' = a << 57
+	or	eax, edx		// a' = a >>> 7
 
 #elif defined(__arm__)
 
-	notimpl
+	mov	r1, r0, ror #7		// easy way
+
+	// even the hard way is fairly easy on arm
+	mov	r3, r0, lsl #25
+	orr	r0, r3, r0, lsr #7	// hard way
 
 #elif defined(__aarch64__)
 
-	notimpl
+	ror	x1, x0, #7		// easy way
+
+	// even the hard way is fairly easy on arm
+	lsl	x3, x0, #57
+	orr	x0, x3, x0, lsr #7	// hard way
 
 #else
 	notimpl
 #endif
 
+	ret
+
 endproc
 
 proc	x27
 
+	// shift a right by c places, in two halves
+
 #if defined(__x86_64__)
 
-	notimpl
+	mov	ch, cl			// c' = [c, c]
+	inc	ch			// c' = [c, c + 1]
+	shr	ch, 1
+	shr	cl, 1			// c' = [floor(c/2), ceil(c/2)]
+	shr	rax, cl
+	xchg	ch, cl
+	shr	rax, cl
 
 #elif defined(__i386__)
 
-	notimpl
+	mov	ch, cl			// c' = [c, c]
+	inc	ch			// c' = [c, c + 1]
+	shr	ch, 1
+	shr	cl, 1			// c' = [floor(c/2), ceil(c/2)]
+	shr	eax, cl
+	xchg	ch, cl
+	shr	eax, cl
 
 #elif defined(__arm__)
 
-	notimpl
+	// it would be clearer and more efficient to say: `mov r12, r2, lsr
+	// #1; sub r2, r2, r12', but that's not the lesson this exercise is
+	// trying to teach.
+	add	r12, r2, #1
+	mov	r2, r2, lsr #1
+	mov	r12, r12, lsr #1
+	mov	r0, r0, lsr r2
+	mov	r0, r0, lsr r12
 
 #elif defined(__aarch64__)
 
-	notimpl
+	add	w16, w2, #1
+	lsr	w2, w2, #1
+	lsr	w16, w16, #1
+	lsr	x0, x0, x2
+	lsr	x0, x0, x16
 
 #else
 	notimpl
 #endif
 
+	ret
+
 endproc
 
 proc	x28
-- 
2.11.0