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Slides for Lecture 6 Previous Lecture Today's Lecture Endianness ENCM 501 W14 Slides for Lecture 6 slide 2/33 Previous Lecture Slides for Lecture 6 ENCM 501: Principles of Computer Architecture Winter 2014 Term I introduction to ISA design ideas Steve Norman, PhD, PEng I memory-register and load-store architectures I a very brief history of RISC versus CISC Electrical & Computer Engineering Schulich School of Engineering I aspects of the ISA view of memory—flat address spaces, University of Calgary alignment rules 28 January, 2014 ENCM 501 W14 Slides for Lecture 6 slide 3/33 ENCM 501 W14 Slides for Lecture 6 slide 4/33 Today’s Lecture Endianness This is not really an aspect of computer design in which there are interesting cost or performance tradeoffs. Rather, it’s an annoying detail that will occasionally bite you if you aren’t I endianness aware of it. I addressing modes Registers inside processor cores do not have endianness. An I examples of tradeoffs in instruction set design N-bit register just has bits N 1 (MSB), N 2, . , 2, 1, 0 (LSB). − − Related reading in Hennessy & Patterson: Sections A.3–A.7 Endianness is a property of the interface between the processor core and the memory, and comes from the fact that most ISAs allow memory reads and writes with various sizes, typically 1-byte, 2-byte, 4-byte, and 8-byte. ENCM 501 W14 Slides for Lecture 6 slide 5/33 ENCM 501 W14 Slides for Lecture 6 slide 6/33 Endianness in 64-bit MIPS doublewords Endianness in 32-bit MIPS words The byte offset gives the address of an individual byte relative The byte offset gives the address of an individual byte relative to the address of the entire doubleword. to the address of the entire word. Bit numbering: 63 is MSB, 0 is LSB Bit numbering: 31 is MSB, 0 is LSB 63 56 55 48 47 40 39 32 31 24 23 16 15 870 31 24 23 16 15 870 +7 +6 +5 +4 +3 +2 +1 +0 +3 +2 +1 +0 LITTLE-endian byte offsets LITTLE-endian byte offsets Bit numbering: 63 is MSB, 0 is LSB Bit numbering: 31 is MSB, 0 is LSB 63 56 55 48 47 40 39 32 31 24 23 16 15 870 31 24 23 16 15 870 +0 +1 +2 +3 +4 +5 +6 +7 +0 +1 +2 +3 BIG-endian byte offsets BIG-endian byte offsets ENCM 501 W14 Slides for Lecture 6 slide 7/33 ENCM 501 W14 Slides for Lecture 6 slide 8/33 Example effect of endianness in MIPS32 Practical code rarely (if ever) writes data as a word and later reads it back as bytes, as was done in the example on the last slide. Why is endianness a practical concern? Assume that R8 contains Here is a practical problem: # LI: pseudoinstruction some valid address that is a I Program P1 on Computer C1 copies an array of integers # for "load immediate" multiple of four. or FP numbers from memory into a file using a function LI R9, 0x12345678 What goes into R10, R11, like fwrite in the C library. SW R9, 0(R8) R12, R13, if the processor I On disk, the file is just a long sequence of bytes. LB R10, 0(R8) chip is in little-endian I Program P2 on Computer C2 opens the file and tries to LB R11, 1(R8) mode? read the array of numbers from the file into memory using LB R12, 2(R8) a function like fread in the C library. LB R13, 3(R8) What if the processor chip is in big-endian mode? I But C2 does not have the same endianness as C1, so the data does not make sense to P2. The same kind of problem can happen when streaming multi-byte numbers over a network. ENCM 501 W14 Slides for Lecture 6 slide 9/33 ENCM 501 W14 Slides for Lecture 6 slide 10/33 Endianness and real systems Addressing modes Today little-endianness is much more common than big-endianness. Here are some little-endian systems: Unlike endianness, selection of addressing modes for an ISA is a set of design decisions that involve interesting tradeoffs. I anything running on x86 or x86-64; I Apple iOS, Linux (including Android), and Windows Addressing mode is a slightly misleading term, because it running on ARM. refers to the way in which an operand is accessed by an Some historically important big-endian machines were: instruction, and that might or might not involve generation of a memory address. I Macs with 68000- or PowerPC-based processors; Addressing modes for data access are discussed as part of I 68000- and SPARC-based computers from Sun Microsystems. Section A.3 in the textbook. Many modern ISA families, for example, MIPS and ARM, Addressing modes for instruction access—needed, for allow the processor to switch back and forth between little- example, by branches and jumps—are discussed in Section A.6. and big-endian modes. ENCM 501 W14 Slides for Lecture 6 slide 11/33 ENCM 501 W14 Slides for Lecture 6 slide 12/33 Examples of addressing modes for data Addressing modes: Register and Immediate Figure A.6 in the textbook gives examples covering most addressing modes available in ISAs of the present and the Register: Data is coming from or going to a register. All recent past. three operands are accessed in register mode in this MIPS64 A typical ISA will support some but not all of these instruction: addressing modes. (Historical note: I think the MC68000 DADDU R10, R8, R9 series supported all of them and more, which is kind of awesome.) Immediate: Source data is a constant written into the instruction. Here is a MIPS64 example in which two operands This lecture won’t explain every addressing mode in detail, but are register-mode and one is immediate-mode: instead will look at the ones that are most common and DADDIU R16, R16, 8 important. Let’s start with the two modes that don’t involve generation of a memory address . ENCM 501 W14 Slides for Lecture 6 slide 13/33 ENCM 501 W14 Slides for Lecture 6 slide 14/33 Encoding of immediate operands in example ISAs The two simplest addressing modes for memory access x86-64: Instruction size is variable, so 1, 2, 4, or 8 bytes are used, as necessary, to describe the constant. Hint for comprehension: Roughly speaking, indirect means “via a pointer”. MIPS32 and MIPS64: Instructions are always 32 bits wide and the field size for immediate operands is always 16 bits Register indirect: Use the bits in a register as a memory wide. The range of constants is 32768 to +32767 for address. MIPS64 example: − instructions that use signed constants and 0 to 65535 for LD R8, (R9) # R8 = doubleword at address in R9 those that use unsigned constants. Displacement: Add a constant to the bits in a register to ARM: 12 bits within the fixed instruction size of 32 bits are generate a memory address. MIPS64 example: used for an immediate operand, in a complicated and # R10 = doubleword at address R10 + 64 bytes interesting way that could totally derail a lecture! (That’s one LD R10, 64(R11) of a few very good reasons why it would not be easy to switch from MIPS to ARM in ENCM 369.) Why is register indirect mode really just a special case of displacement mode? ENCM 501 W14 Slides for Lecture 6 slide 15/33 ENCM 501 W14 Slides for Lecture 6 slide 16/33 Scaled mode: Good for array element access Autoincrement and autodecrement modes (1) Here is some x86-64 assembly language code you will look at Other names for these modes are post-increment and in Assignment 2 . pre-decrement. .L16: In either of these modes a load causes two register mov (%rbx,%rax,4), %edx updates—one to a destination register, and another to a addq $1, %rax pointer register. A store also causes two updates—one update addq %rdx, %rbp to a memory location and another to a pointer register. cmpq $500000000, %rax jne .L16 Both are useful for walking through arrays using pointer arithmetic. The mov instruction uses scaled mode: The address used to A store using pre-decrement mode is an efficient way to read memory is push a register value on to a stack. %rbx + 4 %rax × And a load using post-increment mode is an efficient way to %rbx is the address of element 0 of an array of 4-byte pop a register value from a stack. elements, and %rax is an index into that array. ENCM 501 W14 Slides for Lecture 6 slide 17/33 ENCM 501 W14 Slides for Lecture 6 slide 18/33 Autoincrement and autodecrement modes (2) Memory indirect mode Example, using syntax from textbook Figure A.6: MOV R0, @(R1) The address in R1 is used to read a second address from memory. That second address is used to read from memory into R0. In a These modes closely match some famously tricky C and C++ typical load/store architecture this would be done with two expressions. instructions: a load followed by another load. Let’s write a couple of C statements that could be each be Another example, using the same syntax: implemented using a single instruction if autoincrement and MOV @(R2), R3 autodecrement modes are available. The address in R2 is used to read a second address from memory. That second address is used to write the data from R3 to memory. In a typical load/store architecture this would be done with two instructions: a load followed by a store.
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