MIPS Architecture
- 2. Data Types and Literals Data types: Instructions are all 32 bits byte(8 bits), halfword (2 bytes), word (4 bytes) a character requires 1 byte of storage an integer requires 1 word (4 bytes) of storage Literals: Numbers entered as is. e.g. 4 Characters enclosed in single quotes. e.g. 'b' Strings enclosed in double quotes. e.g. "A string"
- 3. Registers • 32 general-purpose registers • register preceded by $ in assembly language instruction two formats for addressing: – using register number e.g. $0 through $31 – using equivalent names e.g. $t1, $sp • special registers Lo and Hi used to store result of multiplication and division – not directly addressable; contents accessed with special instruction mfhi ("move from Hi") and mflo ("move from Lo") • stack grows from high memory to low memory
- 4. Register Number Alternative Name Description 0 zero the value 0 1 $at (assembler temporary) reserved by the assembler 2-3 $v0 - $v1 (values) from expression evaluation and function results 4-7 $a0 - $a3 (arguments) First four parameters for subroutine. Not preserved across procedure calls 8-15 $t0 - $t7 (temporaries) Caller saved if needed. Subroutines can use w/out saving. Not preserved across procedure calls
- 5. 16-23 $s0 - $s7 (saved values) - Call saved. A subroutine using one of these must save original and restore it before exiting. Preserved across procedure calls 24-25 $t8 - $t9 (temporaries) Caller saved if needed. Subroutines can use w/out saving. These are in addition to $t0 - $t7 above. Not preserved across procedure calls. 26-27 $k0 - $k1 reserved for use by the interrupt/trap handler 28 $gp global pointer. Points to the middle of the 64K block of memory in the static data segment. 29 $sp stack pointer Points to last location on the stack. 30 $s8/$fp saved value / frame pointer Preserved across procedure calls
- 6. Program Structure • Data Declarations – placed in section of program identified with assembler directive .data – declares variable names used in program; storage allocated in main memory (RAM) • Code – placed in section of text identified with assembler directive .text – contains program code (instructions) – starting point for code e.g.excution given label main: – ending point of main code should use exit system call
- 7. • Comments • anything following # on a line # This stuff would be considered a comment
- 8. Data Declarations • format for declarations: name: storage_type value(s) create storage for variable of specified type with given name and specified value – value(s) usually gives initial value(s); – for storage type .space, gives number of spaces to be allocated • Storage_type : word, byte,half word
- 9. Examples of Data Declarations • var1: .word 3 # create a single integer variable with initial value 3 • array1: .byte 'a','b' # create a 2-element character array with elements initialized # to a and b • array2: .space 40 # allocate 40 consecutive bytes, with storage uninitialized # could be used as a 40-element character array, or a # 10-element integer array; a comment should indicate which!
- 10. Load / Store Instructions • Load / Store Instructions – Memory access only allowed with load and store instructions – all other instructions use register operands • load: lw register_destination, Memory # copy word (4 bytes) at source RAM location to destination register. lb register_destination, Memory #copy byte at source RAM location to low-order byte of destination register,and sign extended to higher-order bytes
- 11. Store Word sw register_source, Memory #store word in source register into RAM destination sb register_source, Memory #store byte (low-order) in source register into RAM destination
- 12. load Immediate li register_destination, value #load immediate value into destination register
- 13. Example of Data Transfer Instruction .data var1: .word 23 .text __start: lw $t0, var1 li $t1, 5 sw $t1, var1
- 14. Arithmetic Instructions • most use 3 operands • all operands are registers; no RAM or indirect addressing • operand size is word (4 bytes) • add $t0,$t1,$t2 # $t0 = $t1 + $t2 add as signed (2's complement) integers • sub $t2,$t3,$t4 # $t2 = $t3 - $t4 • addi $t2,$t3, 5 # $t2 = $t3 + 5 "add immediate" (no sub immediate) • addu $t1,$t6,$t7 # $t1 = $t6 + $t7; add as unsigned integers • subu $t1,$t6,$t7 # $t1 = $t6 + $t7; subtract as unsigned integers
- 15. Control Structures • Branches • comparison for conditional branches is built into instruction • b target # unconditional branch to program label target • beq $t0,$t1,target # branch to target if $t0 = $t1 • blt $t0,$t1,target # branch to target if $t0 < $t1 • ble $t0,$t1,target # branch to target if $t0 <= $t1 • bgt $t0,$t1,target # branch to target if $t0 > $t1 • bge $t0,$t1,target # branch to target if $t0 >= $t1 • bne $t0,$t1,target # branch to target if $t0 <> $t1
- 16. • j target # unconditional jump to program label target • jr $t3 # jump to address contained in $t3 ("jump register")