110/15/2004 CSE378 ISA evolution 1Evolution of ISA’s• ISA’s have changed over computer “generations”.• A traditional way to look at ISA complexity encompasses:– Number of addresses per instruction– Regularity/size of instruction formats– Number of addressing types10/15/2004 CSE378 ISA evolution 2Number of addresses per instruction• First computer: 1 memory address + implied accumulator• Then 1 memory address + “index” registers (for addressing operands)• Followed by 1 memory address + “general registers” (for addressing and storing operands)• Then 2 or 3 memory addresses + general registers• Then N memory addresses + general registers (VAX)• Also “0-address” computers, or “stack computers”• In which category is MIPS, and more generally RISC machines?10/15/2004 CSE378 ISA evolution 3Addresses per instruction• RISC machines– Load-store and branches: 1 memory address + 2 registers– All other 3 registers or 2 registers + immediate• CISC machines – Most of them: Two addresses ( destination ← source op. destination)– One operand is a register; other is either register, immediate, or given by memory address– Some special instructions (string manipulation) can have two memory addresses10/15/2004 CSE378 ISA evolution 4Regularity of instruction formats• Started with fixed format (ease of programming in “machine language”; few instructions)• Then more flexibility (assembler/compiler): three or four instruction formats, not necessarily the same length• Then strive for memory compactness. Complex, powerful, variable length instructions (x86)• Back to regular instruction sets: few formats, instructions of the same length (memory is cheap; instructions must be decoded fast)10/15/2004 CSE378 ISA evolution 5Number of instruction formats• RISC: three or four (instructions have same length)• CISC– Several formats, each of fixed (but maybe different) length– Variable length instructions (depends on opcode, addressing of operands etc. Intel x86 instructions from 1 to 17 bytes)• Instruction encoding via “specifiers”10/15/2004 CSE378 ISA evolution 6Addressing modes• In early machines: immediate, direct, indirect• Then index registers• Then index + base (sum of 2 registers instead of -- or in addition to -- index + displacement)• All kinds of additional modes (indirect addressing, auto-increment, combinations of the above etc.)• In general RISC– Immediate, indexed, and sometimes index + base (IBM Power PC)• CISC– Anything goes...210/15/2004 CSE378 ISA evolution 7The Ultimate CISC - VAX-11• ISA defined late 70’s. Last product mid 80’s• Over 200 instructions– Some very powerful: “polynomial evaluation”, procedure calls with register saving and frame set-up etc• Complex addressing modes10/15/2004 CSE378 ISA evolution 8A sample of VAX addressing modes• Immediate (with even some small f-p constants)• Direct (register) One instruction for each I-unit type• Indirect (deferred)• Autodecrement (and autoincrement) . The register is incremented by the I-unit type before (after) the operand is accessed• Displacement (like MIPS indexed)• Index like displacement but offset depends on the I-unit• Combination of the above and more10/15/2004 CSE378 ISA evolution 9Examples• CLRL register (clears a whole register)• CLRB register (clears the low byte of the register)• CLRL (register) clears memory loc. whose add. is in reg.• CLRL (register)+ as above but then register is incr. by 4• CLRL @(register)+ as above with 1 more level of indirection (register points to address of address)• CLRL offset(register) offset mult.by 4 for L, by 1 for B etc• CLRL offset[register] similar but use offset + 4 * register• CLRL 12(R4)+[R1] clear word at add. R4 + 12*4 + R1*4 and add 4 to R410/15/2004 CSE378 ISA evolution 10Intel x86: the largest number of CPU’s in the world• ISA defined early 80’s• Compatibility hurts: – 16 to 32-bit architecture (64-bit has been announced)– Paucity of general-purpose registers -- only 8• Addressing relies on segments (code, data, stack)• Lots of different instruction formats• Lots of addressing modes (less than the Vax though)• But … over 400(?) millions CPU’s in the world and growing– 90% (?)of the market if you don’t count embedded processors10/15/2004 CSE378 ISA evolution 11X86 instruction encoding• Opcode 1 or 2 bytes (defined by one bit in first byte)• First byte following opcode is operand specifier– e.g., 2 registers– 1 register and the next byte specifies base and index register for a memory address for second operand and next byte specifies a displacement etc– etc.• No regularity in instruction set 10/15/2004 CSE378 ISA evolution 12MIPS is not the only RISC• MIPS family outgrowth of research at Stanford (Hennessy)• DEC (Compaq,HP) Alpha had its roots in MIPS– Alas, discontinued• Sun family outgrowth of research at Berkeley (Patterson)• IBM Power/PC family outgrowth of research at IBM Watson (Cocke)• HP Precision architecture• more ...310/15/2004 CSE378 ISA evolution 13Recent trends for high-end servers• 32-bit architectures become 64-bit architectures– already in Dec Alpha, some HP-PA, in a future generation of Intel x86 (now called IA-32)• A “new” type of instruction format– VLIW (Very Long Instruction Word) or EPIC (Explicitly Parallel Instruction Computing)• Intel-HP Itanium(IA-64)• Multithreaded architectures (Tera, SMT is a UW invention; Hyperthreading in some recent Intel processors)• More than one processor on a chip – CMP (IBM Power 4)• Embedded systems become “systems on a chip”10/15/2004 CSE378 ISA evolution 14Current trends in RISC• Not that “restricted”– instructions for MMX (multimedia)– instructions for multiprocessing (synchronization, memory hierarchy)– instructions for graphics• Design is becoming more complex • Execute several instructions at once (multiple ALU’s)– Speculative execution (e.g., guess branch outcomes)– Execute instructions out-of-order • Ultimate goal is
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