mag1cianag / cs61c

ucb cs61c

cs61c

ucb cs61c sp20

Unconditional Control Instructions:

jal: Unconditional jump and link
ex: jal x3 ,label
jump target specified as label
label is encoded as an offset from current instruction
link the return address

jalr: Unconditional jump via register and link
ex: jalr x3,4(x1)
Jump targt specified as register value plus constant offset
can jump to any 32 bit address - supports long jumps

Pseudoinstruction:

mv x2, x1
li x2, 3
ble x1, x2, label
beqz x1, label
bnez x1, label
j label

Equivalent Assembly Instruction:

addi x2, x1, 0
addi x2, x0, 3
bge x2, x1, label
beq x1, x0, label
bne x1, x0, label
jal x0, label

In risc-v, small constants(12-bit) can be handled via register Immediate ALR operations.(addi x1, x2,3)

Largest 12 bit 2's complement constant is 2^11-1 = 2047(0x7FF) so use li pseudoinstruction to set register to large constant
ex: li x4,0x123456 (equals lui x4,0x123
addi x4,x4,0x456)
can also use li pseudoinstruction for small constants

if statement can be compiled using branches:

C code               RISC-V Assembly
if(expr) {           (compile expr into xN)
  if-body            beqz xN, endif
}                     (compile if-body)
                    endif:

ex:

int x,y;
if(x<y){
    y = y - x;
}
as //x: x10,y:x11

slt x12,x10,x11
beqz x12 ,endif
sub x11, x11 ,x10
endif:

combine expr and the branch

bge x10,x11,endif
sub x11,x11,x10
endif:

Loops can be compiled using backward branches:

C
while(expr){
    while-body
}

RISC-V
while:
  (compile expr into xN)
  beqz xN,endwhile
  (compile while-body)
  j while
endwhile:

With one branch or jump per iteration
j compare
loop:
    (compile while-body)
compare:
    (compile expr into xN)
    benz xN, loop

ex:
while(x!=y){
    if(x>y){
        x = x-y;
    }else {
        y = y-x;
    }
}

//x: x10 ,y: x11
j compare
loop:
    ble x10,x11,else
    sub x10,x10,x11
    j endif
else:
    sub x11,x11,x10
endif:
compare:
    bne x10,x11,loop

proc_call:jal ra,label
1.stores address of proc_call + 4 in register ra(return address register)
2.Jumps to instruction at address label where label is the name of the procedure
3.After excuting procedure,jr ra to return to caller and continue execution

pesudo return from subroutine

Each procedure call has its own instance of all this data known as the procedure's activation record.

RISC-V calling convention gives symbolic names to registers x0-x31

a0 to a7   function arguments caller
a0 and a1 function return values caller
ra x1 return address caller
t0 to t6 temporaries caller
s0 to s11 saved registers callee 
sp x2 stack pointer callee
gp x3 global pointer
tp x4 thread pointer
zero x0 hardwired zero