MIPS (SPIM) Assembler Syntax

1. MIPS (SPIM) Assembler Syntax • Comments begin with #. Everything from # to the end of the line is ignored. • Identifiers are a sequence of alphanumeric characters, underbars (_), and dots (.) that do not begin with a number. • Labels are declared by putting them at the beginning of a line followed by a colon. .data item: .word 1 .text .global main # Must be global main: lw $t0, item

2. SPIM supported MIPS directive • .alignn align the next datum on a 2n byte boundary. • .ascii str store the string str in mem, but do not null-terminate it. • .asciiz str store the string str in mem, but null- terminate it. • .byte b1, …, bn store the n values in successive bytes of memory. • .data <addr> subsequent items are stored in the data segment. If the optional argument addr is present, subsequent items are stored starting at address addr. • .double d1,…,dn store the n floating-point double precision numbers in successive memory locations.

3. SPIM supported MIPS directive (cont’d) • .extern sym size Declare that the datum stored at sym is size bytes large and is a global label. • .float f1,…,fn store the n floating-point single precision numbers in successive memory locations. • .global sym Declare that label sym is global and can be referenced from other files. • .half h1, …, hn store the n 16-bit quantities in successive memory halfwords. • .kdata <addr> subsequent items are stored in the kernel data segment. If the optional argument addr is present, subsequent items are stored starting at addr. • .ktext <addr> Subsequent items are put in the kernel text segment. If the optional argument addr is present, subsequent items are stored starting at addr.

4. SPIM supported MIPS directive (cont’d) • .set no at It prevents SPIM from complaining about subsequent instructions that use register $at. • .set no at It prevents SPIM from complaining about subsequent instructions that use register $at. • .space n Allocate n bytes of space in the current segment (which must be data segment in SPIM) • .text <addr> Subsequent items are put in the text segment. If the optional argument addr is present, subsequent items are stored starting at addr. • .word w1,…,wn store the n 32-bit quantities in successive memory words.

5. Branching Far Away • What if the branch destination is further away than can be captured in 16 bits? beq $s0, $s1, L1

6. Dealing with Constants • Small constants are used quite frequently (often 50% of operands) e.g., A = A + 5; B = B + 1; C = C - 18; • Solutions? Why not? • put “typical constants” in memory and load them • create hard-wired registers (like $zero) for constants • Allow for MIPS instructions like addi $sp, $sp, 4 slti $t0, $t1, 10 andi $t0, $t0, 6 ori $t0, $t0, 4 • How do we make this work?

7. Immediate Operands • MIPS immediate instructions:addi $sp, $sp, 4 #$sp = $sp + 4 slti $t0, $s2, 15 #$t0 = 1 if $s2<15 • Machine format: • The constant is kept inside the instruction itself! • I format – Immediate format • Limits immediate values to the range +215–1 to -215 I format op rs rt 16 bit immediate 8 29 29 4 10 18 8 15

8. How About Larger Constants? • We'd also like to be able to load a 32 bit constant into a register • Must use two instructions, new "load upper immediate" instruction lui $t0, 1010101010101010 • Then must get the lower order bits right, i.e., ori $t0, $t0, 1010101010101010 16 0 8 1010101010101010 1010101010101010 0000000000000000 0000000000000000 1010101010101010

9. MIPS Addressing Modes • Register addressing – operand is in a register • Base (displacement) addressing – operand is at the memory location whose address is the sum of a register and a 16-bit constant contained within the instruction • Immediate addressing – operand is a 16-bit constant contained within the instruction • PC-relative addressing –instruction address is the sum of the PC and a 16-bit constant contained within the instruction • Pseudo-direct addressing – instruction address is the 26-bit constant contained within the instruction concatenated with the upper 4 bits of the PC

10. 1. Register addressing op rs rt rd funct Register word operand 2. Base addressing op rs rt offset Memory word or byte operand base register 3. Immediate addressing op rs rt operand 4. PC-relative addressing op rs rt offset Memory branch destination instruction Program Counter (PC) 5. Pseudo-direct addressing Memory op jump address || jump destination instruction Program Counter (PC) Addressing Modes Illustrated

11. Design Principles • Simplicity favors regularity • fixed size instructions – 32-bits • small number of instruction formats • Smaller is faster • limited instruction set • limited number of registers in register file • limited number of addressing modes • Good design demands good compromises • three instruction formats • Make the common case fast • arithmetic operands from the register file (load-store machine) • allow instructions to contain immediate operands

12. Review: MIPS ISA, so far

13. compiler assembly code assembler object code library routines linker machine code executable loader memory The Code Translation Hierarchy C program

14. Compiler • Transforms the C program into an assembly language program • Advantages of high-level languages • many fewer lines of code • easier to understand and debug • Today’s optimizing compilers can produce assembly code nearly as good as an assembly language programming expert and often better for large programs • good – smaller code size, faster execution

15. Assembler • Transforms symbolic assembler code into object (machine) code • Advantages of assembly language • Programmer has more control compared to higher level language • much easier than remembering instruction binary codes • can use labels for addresses – and let the assembler do the arithmetic • can use pseudo-instructions • e.g., “move $t0, $t1” exists only in assembler (would be implemented using “add $t0,$t1,$zero”) • However, must remember that machine language is the underlying reality • e.g., destination is no longer specified first • And, when considering performance, you should count real instructions executed, not code size

16. Other Tasks of the Assembler • Determines binary addresses corresponding to all labels • keeps track of labels used in branches and data transfer instructions in a symbol table • pairs of symbols and addresses • Converts pseudo-instructions to legal assembly code • register $at is reserved for the assembler to do this • Converts branches to far away locations into a branch followed by a jump • Converts instructions with large immediates into a load upper immediate followed by an or immediate • Converts numbers specified in decimal and hexidecimal into their binary equivalents • Converts characters into their ASCII equivalents

17. Typical Object File Pieces • Object file header: size and position of following pieces • Text module: assembled object (machine) code • Data module: data accompanying the code • static data - allocated throughout the program • dynamic data - grows and shrinks as needed by the program • Relocation information: identifies instructions (data) that use (are located at) absolute addresses – those that are not relative to a register (e.g., jump destination addr) – when the code and data is loaded into memory • Symbol table: remaining undefined labels (e.g., external references) • Debugging information Object file header text segment data segment relocation information symbol table debugging information

18. MIPS (spim) Memory Allocation Memory f f f f f f f c Mem Map I/O Kernel Code & Data 8000 0080 $sp 7f f e f f fc Stack 230 words Dynamic data $gp 1000 8000 ( 1004 0000) Static data 1000 0000 Your Code PC 0040 0000 Reserved 0000 0000

19. Process that produces an executable file Source file object file compiler + assembler Source file object file Executable file linker compiler + assembler program library object file Source file compiler + assembler

20. Linker • Takes all of the independently assembled code segments and “stitches” (links) them together • Much faster to patch code and recompile and reassemble that patched routine, than it is to recompile and reassemble the entire program • Decides on memory allocation pattern for the code and data modules of each segment • remember, segments were assembled in isolation so each assumes its code’s starting location is 0x0040 0000 and its static data starting location is 0x1000 0000 • Absolute addresses must be relocated to reflect the new starting location of each code and data module • Uses the symbol table information to resolve all remaining undefined labels • branches, jumps, and data addresses to external segments

21. Loader object file object file sub: . . . executable file main: jal ??? . . . jal ??? call, sub call, printf main: jal sub . . . jal printf printf: . . . sub: . . . instructions linker Relocation records C library printf: . . .

22. Loader • Loads (copies) the executable code now stored on disk into memory at the starting address specified by the operating system • Initializes the machine registers and sets the stack pointer to the first free location (0x7ffe fffc) • Copies the parameters (if any) to the main routine onto the stack • Jumps to a start-up routine (at PC addr 0x0040 0000 on xspim) that copies the parameters into the argument registers and then calls the main routine of the program with a jal main