Compiling examples in MIPS Compiling of one module The compiler’s task is to create the machine instructions in correspondence to the existing assembly language instructions.

Compiler prepares & “maps” – doesn’t run .text main: ori $3,$0,1 sltiu $8,$2,56 beq $8,$0,out nop sltiu $8,$2,30 beq $8,$0,cont out: ori $3,$0,0 cont: nop [0x00400000] ?? ?? ?? ?? [0x00400004] [0x00400008] [0x0040000c] [0x00400010] [0x00400014] [0x00400018] [0x0040001c] [0x00400020] . . . Compiler’s task is not to put the machine instructions to the memory addresses but to prepare the machine instructions content supposing that later those instructions will be placed in the memory by OS starting from the address 0x00400000 and then should run. Compiler creates the map of memory. We have declared “.text” segment so our program will start from the default address 0x00400000 and the first machine instruction will be placed in that address when the program will run. Declaration of “.text 0x00500000” supposes that the program will be placed on that address to run

Some instructions are translated immediately some of them - not [0x00400000] 0x34030001 ori $3, $0, 1 [0x00400004] [0x00400008] [0x0040000c] [0x00400010] [0x00400014] [0x00400018] [0x0040001c] [0x00400020] .text main: ori $3,$0,1 sltiu $8,$2,56 beq $8,$0,out nop sltiu $8,$2,30 beq $8,$0,cont out: ori $3,$0,0 cont: nop ori $3,$0,1 could be translated to the machine instruction immediately MIPS has 3 types of machine instructions format and 5 types of addressing modes Some assembly instruction types and some addressing modes immediately could be translated to the machine instructions.

What to do with the main ? Symbol table of this module [0x00400000] 0x34030001 ori $3, $0, 1 [0x00400004] [0x00400008] [0x0040000c] [0x00400010] [0x00400014] [0x00400018] [0x0040001c] [0x00400020] .text main: ori $3,$0,1 sltiu $8,$2,56 beq $8,$0,out nop sltiu $8,$2,30 beq $8,$0,cont out: ori $3,$0,0 cont: nop Symbol table of this module Label Type Address main global 0x00400000 . . . All labels (instructions and data) are collected in “Symbol table” to be used later by compiler or linker to resolve the addresses of that labels. Symbol table belongs to this object module and is kept together with the object module to be used later

The next instruction So far so good sltiu $8,$2,56 [0x00400000] 0x34030001 ori $3, $0, 1 [0x00400004] 0x2c480038 sltiu $8, $2, 56 [0x00400008] [0x0040000c] [0x00400010] [0x00400014] [0x00400018] [0x0040001c] [0x00400020] .text main: ori $3,$0,1 sltiu $8,$2,56 beq $8,$0,out nop sltiu $8,$2,30 beq $8,$0,cont out: ori $3,$0,0 cont: nop So far so good sltiu $8,$2,56 translated to the machine instruction without dependencies and takes the next word

Branch – unresolved address [0x00400000] 0x34030001 ori $3, $0, 1 [0x00400004] 0x2c480038 sltiu $8, $2, 56 [0x00400008] 0x1100xxxx beq $8, $0, out ??? [0x0040000c] [0x00400010] [0x00400014] [0x00400018] [0x0040001c] [0x00400020] Symbol table of this module Label Type Address main global 0x00400000 . . . .text main: ori $3,$0,1 sltiu $8,$2,56 beq $8,$0,out nop sltiu $8,$2,30 beq $8,$0,cont out: ori $3,$0,0 cont: nop Temporary unresolved Symbol Table out – 0x0040 0008 beq $8,$0,out Only the first half of the instruction is known. We don’t know the address of “out” to fill the 2nd half (look at symbol table). We remember this gap in temporary symbol table (???).

No dependency is always easy to compile [0x00400000] 0x34030001 [0x00400004] 0x2c480038 [0x00400008] 0x1100xxxx beq $8, $0, out ??? [0x0040000c] 0x00000000 nop [0x00400010] 0x2c48001e sltiu $8, $2, 30 [0x00400014] [0x00400018] [0x0040001c] [0x00400020] .text main: ori $3,$0,1 sltiu $8,$2,56 beq $8,$0,out nop sltiu $8,$2,30 beq $8,$0,cont out: ori $3,$0,0 cont: nop

Again Branch – unresolved address .text main: ori $3,$0,1 sltiu $8,$2,56 beq $8,$0,out nop sltiu $8,$2,30 beq $8,$0,cont out: ori $3,$0,0 cont: nop [0x00400000] 0x34030001 [0x00400004] 0x2c480038 [0x00400008] 0x1100xxxx [0x0040000c] 0x00000000 [0x00400010] 0x2c48001e [0x00400014] 0x1100xxxx [0x00400018] [0x0040001c] [0x00400020] Temporary unresolved Symbol Table out – 0x0040 0008 cont- 0x0040 0014 beq $8,$0,cont We don’t know the address of “cont” to fill the 2nd half (look at symbol table). We remember this gap also in temporary symbol table. Symbol table of this module Label Type Address main global 0x00400000 . . .

First pass is finished .text main: ori $3,$0,1 sltiu $8,$2,56 beq $8,$0,out nop sltiu $8,$2,30 beq $8,$0,cont out: ori $3,$0,0 cont: nop [0x00400000] 0x34030001 [0x00400004] 0x2c480038 [0x00400008] 0x1100xxxx [0x0040000c] 0x00000000 [0x00400010] 0x2c48001e [0x00400014] 0x1100xxxx [0x00400018] 0x00000000 [0x0040001c] 0x34030000 [0x00400020] 0x00000000 Now we know the addresses and can fill the gaps. Go to second pass. nop – 0s out – Should be added to the Symbol Table. cont – Should be added to the Symbol Table. Symbol table of this module Label Type Address main global 0x00400000 out local 0x0040001c cont local 0x00400020

Second pass, fill the gaps from symbol table. .text main: ori $3,$0,1 sltiu $8,$2,56 beq $8,$0,out nop sltiu $8,$2,30 beq $8,$0,cont out: ori $3,$0,0 cont: nop [0x00400000] 0x34030001 [0x00400004] 0x2c480038 [0x00400008] 0x11000004 [0x0040000c] 0x00000000 [0x00400010] 0x2c48001e [0x00400014] 0x11000002 [0x00400018] 0x00000000 [0x0040001c] 0x34030000 [0x00400020] 0x00000000 +16 +8 Our program is ready to load to memory and run from 0x00400000 address Symbol table of this module (kept) Label Type Address main global 0x00400000 out local 0x0040001c cont local 0x00400020 Temporary unresolved Symbol Table out – 0x0040 0008 cont- 0x0040 0014 Can this program work if it will be loaded to the memory another address? Temporary unresolved Symbol Table eliminated out – 0x0040 0008 cont- 0x0040 0014 Yes. The addresses are relative.