From 98c141acc95bf183b4231c270aded680f7105b9f Mon Sep 17 00:00:00 2001
From: FreeArtMan <dos21h@gmail.com>
Date: Tue, 29 Jun 2021 07:24:03 +0100
Subject: arm64 some fixes and publish on main page

---
 md/writeup/arm64_assembly_crc32.md | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

(limited to 'md/writeup')

diff --git a/md/writeup/arm64_assembly_crc32.md b/md/writeup/arm64_assembly_crc32.md
index c284349..48ffd21 100644
--- a/md/writeup/arm64_assembly_crc32.md
+++ b/md/writeup/arm64_assembly_crc32.md
@@ -12,6 +12,9 @@ that functionality for macos and linux.
 
 ## Verify architecture
 
+Before running any of this code, check that archirecture is correct in case of Apple M1 its easiest to do.
+With Raspberry Pi 4 need to run it with 64bit linux.
+
 ### Raspberry Pi 4
 
 Run command
@@ -19,13 +22,13 @@ Run command
 uname -a
 Linux raspberrypi 5.4.42-v8+ #1319 SMP PREEMPT Wed May 20 14:18:56 BST 2020 aarch64 GNU/Linux
 ```
-There is substring "aarch64" that indicates that os bitnes is 64bit 
+There is substring "aarch64" that indicates that os supports 64 bits 
 
 To check if CPU supports crc32 instructions run
 ```
 cat /proc/cpuinfo | grep crc
 ```
-check for "crc32" substring
+search for "crc32" substring
 
 ### Apple M1
 
@@ -45,16 +48,18 @@ in output string should be "arm64"
 ## ARM64 crc32 instructions
 
 Wiki have table with all different kind of implementation of crc32 https://en.wikipedia.org/wiki/Cyclic_redundancy_check 
-arm64 crc32 implementation reffered in wiki table as __CRC-32__. There is support also for __CRC-32C__.
+arm64 crc32 implementation reffered in wiki table as __CRC-32__. There is support also for __CRC-32C__ variant.
 
 There is 4 types of crc32 instruction for 1,2,4,8 byte of data that are marked with
 postfix b,h,w,x.
 Wraped up examples of instructions:
 ```c
+//crc32
 #define CRC32X(crc, value) __asm__("crc32x %w[c], %w[c], %x[v]":[c]"+r"(crc):[v]"r"(value))
 #define CRC32W(crc, value) __asm__("crc32w %w[c], %w[c], %w[v]":[c]"+r"(crc):[v]"r"(value))
 #define CRC32H(crc, value) __asm__("crc32h %w[c], %w[c], %w[v]":[c]"+r"(crc):[v]"r"(value))
 #define CRC32B(crc, value) __asm__("crc32b %w[c], %w[c], %w[v]":[c]"+r"(crc):[v]"r"(value))
+//crc32c
 #define CRC32CX(crc, value) __asm__("crc32cx %w[c], %w[c], %x[v]":[c]"+r"(crc):[v]"r"(value))
 #define CRC32CW(crc, value) __asm__("crc32cw %w[c], %w[c], %w[v]":[c]"+r"(crc):[v]"r"(value))
 #define CRC32CH(crc, value) __asm__("crc32ch %w[c], %w[c], %w[v]":[c]"+r"(crc):[v]"r"(value))
@@ -101,6 +106,10 @@ initial values is __crc=0x0__ and final result not need to be preprocessed.
 
 
 ## Source code
+
+Here is complete source code of software and hardware crc32, that runs over some array of data and compares
+results of 2 software implementations and hardware.
+
 ```c
 #include <stdlib.h>
 #include <stdio.h>
@@ -218,13 +227,13 @@ int main() {
 ## Compile
 
 ### Raspbery Pi 4
-
+Need to set extra *-march* option to enable architecture variant that supports crc.
 ```
 gcc asm_crc32.c -o asm_crc32 -march=armv8.1-a
 ```
 
 ### Apple M1
-
+Works without extra options
 ```
 gcc asm_crc32.c -o asm_crc32
 ```
-- 
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