CISC versus RISC

1
CISC versus RISC

• Two competing philosophies for the design of a
  processors instruction-set

2
The RISC idea

• RISC Reduced Instruction-Set Computer
• There will be a relatively small number of very
  simple operations that the CPU can execute, which
  the programmer uses as basic building blocks for
  constructing complex algorithms
• Because there are so few instructions, they can
  be uniquely identified by only a few bits
• Hence these short instructions can be more
  quickly fetched, decoded, and executed

3
The CISC idea

• CISC Complex Instruction-Set Computer
• Identify the most important operations that a
  computer will need to perform, then build the
  circuitry for each one of those operations into
  its own special-purpose machine-instruction
• Instead of fetching a large number of small
  instructions, it will be more efficient to fetch
  and execute just one very powerful instruction

4
Some x86 CISC instructions

• The x86s string-manipulation instructions are
  examples of the CISC philosophy
• These instructions are specially designed to do
  the most common word processing operations in
  an optimized manner (e.g., copying, comparing,
  and searching long strings of characters stored
  in memory)

5
The string operations

• Move-string movs
• Compare-strings cmps
• Scan-string scas
• Store-string stos
• Load-string lods
• Output-string outs
• Input-string ins

6
Initial setup

• Each of the string-instructions requires a
  certain amount of preparatory setup, but then can
  repeatedly execute an operation without needing
  to re-fetch and re-decode
• Hence the overall string-operation can be done
  more efficiently than could be done by using a
  customary program-loop

7
Example strlen
Finding the length of a null-terminated
string of ASCII character-codes mov msg,
rdi point RDI to beginning of the
string mov 0, al setup the null-bytes
value in AL cld clear the DF-bit in EFLAGS
mov -1, rcx maximum unsigned integer in
RCX repne scasb scan string for its first
NULL byte sub msg, rdi subtract start to
get string length
8
Midterm question

• On a different topic, we show a slide which
  conveys an idea for solving Question V
• Most of the code in our myargs.s demo can be
  reused, but a couple of changes will be needed
• Start from the end of the array of pointers,
  instead of from the beginning
• Decrement the array-index upon each pass through
  the loop, instead of incrementing it

9
reorder.s
command-line
./reorder xx yyy zzzz
0 8 16
24 32 40
argc (4)
argv0
argv1
argv2
argv3
NULL (0)
RSP
.
/
r
e
o
r
d
e
r
x
x
y
y
y
z
z
z
z
argc (argument count) number of strings
typed on the command-line
mov (rsp), rbx dec rbx mov 8( rsp,
rbx, 8 ), rsi
? RSI (argv3)