1. Computer Architecture and System Programming Laboratory
TA Session 12
x86-SSE text string processing instructions

2. X86-SSE Programming – Text Strings (SSE4.2)
An implicit-length text string uses a terminating End-Of-String (EOS) character.
X86-SSE includes four SIMD text string instructions that are capable of processing
text string fragments up to 128 bits in length.
Suppose you are given a text string fragment and want to create a mask to indicate the
positions of the uppercase characters within the string.
For example, each 1 in the mask b signifies an uppercase character in the corresponding position of the text string "Ab1cDE23f4gHi5J6".
The desired character range and text string fragment are loaded into registers XMM1 and XMM2, respectively.

3. RFLAGS:
0x4831 = b

4. RFLAGS: RFLAGS: RFLAGS:
the output format bit 6 is set, which means that the mask value is expanded to bytes
RFLAGS:
multiple character ranges
XMM1 contains two range pairs: one for uppercase letters and one for lowercase letters.
RFLAGS:
text string fragment that includes an embedded EOS (‘\0’) character
ZF is set to 1
final mask value excludes matching range characters following EOS

5. CF flag – Reset if IntRes2 is equal to zero, set otherwise
RFLAGS:
multiple character ranges
XMM1 contains two range pairs: one for uppercase letters and one for lowercase letters.
RFLAGS is set in a non-standard manner in order to supply the most relevant information:
CF flag – Reset if IntRes2 is equal to zero, set otherwise
ZF flag – Set if any byte/word of xmm2/mem128 is null, reset otherwise
SF flag – Set if any byte/word of xmm1 is null, reset otherwise
OF flag – IntRes2[0]
AF flag – Reset
PF flag – Reset

6. AZ2az_mask: times 16 db ('a' - 'A’) result: times 16 db 0 db `\n\0`
section .data
str: db ‘Ab1cDE23f4gHi5J6’
AZ_mask: db ‘A', ‘Z’
times 14 db 0
imm: equ b
AZ2az_mask: times 16 db ('a' - 'A’)
result: times 16 db 0
db `\n\0`
extern printf
section .text
global main
main:
enter
movdqu xmm1, [AZ_mask]
movdqu xmm2, [str]
pcmpistrm xmm1, xmm2, imm
movdqu xmm3, [AZ2az_mask]
pand xmm0, xmm3
paddb xmm2, xmm0
movdqu [result], xmm2
mov rdi, result
mov rax, 0
call printf
leave
ret
MOVDQU xmm1, xmm2/m128
Move unaligned double quadword from xmm2/m128 to xmm1.
PADDB xmm1, xmm2/m128
Add packed byte integers from xmm2/m128 and xmm1.
PAND xmm1, xmm2/m128
Bitwise AND of xmm2/m128 and xmm1.

7. Equal any (imm[3:2] = 00).
The result is a bit mask – 1 if the character belongs to a set, 0 if not.
pcmpstrim xmm1, xmm2, b 00
‘\0’
‘1’
‘k’
‘b’
‘a’
‘2’
xmm1 00
‘\0’
‘1’
‘k’
‘C’
‘a’
xmm2 00 FF
xmm0
Equal each (imm[3:2] = 10). The result is a bit mask – 1 if the corresponding bytes are equal, 0 if not equal.
pcmpstrim xmm1, xmm2, b 00
‘\0’
‘1’
‘k’
‘b’
‘a’
xmm1 00
‘\0’
‘1’
‘k’
‘C’
‘a’
xmm2 00 FF
xmm0

8. Equal ordered (imm[3:2] = 11).
The result is a bit mask – 1 if the substring is found at the corresponding position, 0 otherwise.
pcmpstrim xmm1, xmm2, b 00
‘e’
‘W’
xmm1
‘!’
‘d’
‘e’
‘W’
‘B’
‘l’
‘i’
‘n’
‘h’
xmm2 00 FF
xmm0

9. RCX = 16 (invalid index) rcx RFLAGS: RCX
IntRes1 calculation – mask according to the given range
bit index in IntRes1 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
bit value in IntRes1
Negative- IntRes2 calculation
bit index in IntRes1 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
bit value in IntRes1
RCX = index of least significant set bit in IntRes2
RCX = 16 (invalid index)
RCX

10. RCX = 11 (index of ‘\0’ character, or length of string)
RFLAGS:
rcx
IntRes1 calculation – mask according to the given range
bit index in IntRes1 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
bit value in IntRes1
Negative- IntRes2 calculation
bit index in IntRes1 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
bit value in IntRes1
RCX = index of least significant set bit in IntRes2
RCX = 11 (index of ‘\0’ character, or length of string)
RCX

11. rcx
RFLAGS:
RCX

12. first loop cycle: second loop cycle: section .data
RFLAGS:
rcx
section .data
str: db ‘Ab1cDE23f4gHi5J6’
db ‘Ab1cDE23f4g\0’
EOS_mask: db 0x1,0xFF
times 14 db 0
imm: equ b
section .text
global strlen
strlen:
enter
xor rax
xor rcx
movdqu xmm1, [EOS_mask]
.loop
add rax, rcx
pcmpistri xmm1, [str+rax], imm
jnz .loop
leave
ret
second loop cycle:
RFLAGS:
rcx

13. RCX