Computer Science 237 Computer Organization Williams College Fall 2005

Announcements

• Be a Computer Science teaching assistant for the spring semester! See Lorraine Robinson in the CS office or fill out the online form to apply. The deadline is November 15.
• Computer Science Alumni Panel this Friday, 2:35 p.m., TCL 202. Come hear what some of our recent alumni are doing with their Computer Science degrees.
• Exam out today after a short microarchitecture lecture.

Agenda

• Tanenbaum's Macroarchitecture
  • Today, these macroarchitectures or macroinstruction sets are called instruction set architectures or ISA's. There are several very successful ISA's competing for world markets today: POWER, IA32, IA64, M68K, MIPS, Alpha, various VLIW (very long instruction word), etc.
  • The instruction set (Fig. 4-14 in Tanenbaum 1990)
    • All instructions are 16 bits wide
    • Memory is only 4K 16 bit words, thus 12 bits of address space. (Address space grows by 1.5 bits per year.)
    • No registers, just an accumulator (AC). A stack pointer, but no frame pointer.
    • Most instructions also carry data--either a 12-bit address (e.g. LODD) or a 12-bit constant (e.g. LOCO)
    • There are more than 16 instructions, so 4 bits is not sufficient. We break out instruction 15 (opcode 1111) into 8 instructions.
    • There are still many instructions available for use. For example any of PSHI through SWAP with bits set in low 9 bits, or INSP/DESP with bit 8 set.
    • Clearly instructions are grouped by function.
    • Instructions with more bits will be harder to decode; they should be used less frequently (an information theory argument can be used here).
  • Translation from PASCAL.
    • Program in Figure 4-11(a) in Tanenbaum 1990 performs the inner product of two 20-element vectors. Notice that arrays are indexed beginning at 1, here. Look for this during indexing.
    • Variables k, x, and y are global, but thought of as local to the main procedure (at the bottom, lines 14-19).
    • Memory is 4,096 16-bit words.

      The compiler places the globals in high memory, below stack (K is at 4020, X is 20 below Y, Y is 20 below X). These fixed values make the code nonrelocatable. It would be impossible to have two of these programs running on the same machine, or a version running in a different location, since the values are hard-wired into the code.

    • Other assignments are setting up the call frame offsets for the routines pmul and inner
    • Notice the direct line-by-line translation puts main at the bottom, but the first instruction jumps to the main procedure.

      This is how a compiler might perform the translation.

    • main: SP starts at 0, DESP 41 allocates a frame for main's locals (41 words worth).
    • Notice calling convention pushes arguments on in order. PASCAL does not have C-like arbitrary parameter semantics (save for system I/O routines, handled in a different manner).
    • pmul: has 2 locals. SOTL supports frame-relative stores, but relative to the SP. Check out call frame offsets.

      

    • Important things to notice: this is a valid assembly language that we would like to implement on top of the MIC1 using microcode to implement each instruction and the processing of those instructions.