Does the “Multiply-Subtract” MLS{cond} Rd, Rn, Rm, Ra instruction modify the condition code flags in ARMv7? The documentation for MUL, MLA, and MLS states that “S is an optional suffix. If S is specified, the condition code flags are updated on the result of the operation.” Is the S in MLS considered a suffix, or is it just part of the name?
For example, in the following code, will the final jump happen when lr != 0, or when (r0 = r0 - r2*r1) != 0?
cmp.w lr, #0x0
...
mls r0, r2, r1, r0
...
bne.w 0x1bc87e
It seems that MLS does not update condition code flags, and there does not exist a version that does (MLSS is invalid).
Related
I don't know if this question is vague or lacks enough information but I was just wondering If I want to convert this line a = a * b * c written in C language to LC-3, how can I do it? Assuming that a, b and c are local variables and that the offset of a is 0, b is -1, c is -2?
I know that I can start off like this:
LDR R0, R5, #0; to load A
LDR R1, R5, #-1; to load B
Is there a limitation to the registers I can use? Can I use R2 to load C?
Edit:
LDR R0, R5, #0; LOAD A
LDR R1, R5, #-1; LOAD B
LDR R2, R5, #-2; LOAD C
AND R3, R3, #0; Sum = 0
LOOP ADD R3, R3, R1; Sum = sum + B
ADD R0, R0, #-1; A = A-1
STR R0, R5, #0; SAVE TO A (a = a*b)
BRp LOOP
ADD R4, R4, R2; Sum1 = sum1 + C
ADD R2, R2, #-1; C = C-1
BRp LOOP
STR R0, R5, #0; SAVE TO A (a = a*c = a*b*c)
If you're writing a whole program, which is often the case with LC-3, the only physical limit is the instruction set, so modulo that, you can use the registers as you like.
Your coursework/assignment may impose some environmental requirements, such as local variables and parameters being accessible from a frame pointer, e.g. R5, and having the stack pointer in R6. If so, then those should probably be left alone, but in a pinch you could save them, and later restore them.
If you're writing just a function that is going to be called, then you'll need to follow the calling convention. Decode the signature of the function you're implementing according to the parameter passing approach. If you want to use R7 (e.g. as a scratch register, or if you want to call another function) be aware that on entry to your function, it holds the return address, whose value is needed to return to the caller, but you can save it on the stack or in a global storage for later retrieval.
The calling convention in use should also inform which registers are call clobbered vs. call preserved. Within a function, call-clobbered registers can be used without fuss, but call-preserved registers require being saving before use, and later restored to original values before returning to the function's caller.
I have a C code in my mind which I want to implement in ARM Programming Language.
The C code I have in my mind is something of this sort:
int a;
scanf("%d",&a);
if(a == 0 || a == 1){
a = 1;
}
else{
a = 2;
}
What I have tried:
//arm equivalent of taking input to reg r0
//check for first condition
cmp r0,#1
moveq r0,#1
//if false
movne r0,#2
//check for second condition
cmp r0,#0
moveq r0,#1
Is this the correct way of implementing it?
Your code is broken for a=0 - single step through it in your head, or in a debugger, to see what happens.
Given this specific condition, it's equivalent to (unsigned)a <= 1U (because negative integer convert to huge unsigned values). You can do a single cmp and movls / movhi. Compilers already spot this optimization; here's how to ask a compiler to make asm for you so you can learn the tricks clever humans programmed into them:
int foo(int a) {
if(a == 0 || a == 1){
a = 1;
}
else{
a = 2;
}
return a;
}
With ARM GCC10 -O3 -marm on the Godbolt compiler explorer:
foo:
cmp r0, #1
movls r0, #1
movhi r0, #2
bx lr
See How to remove "noise" from GCC/clang assembly output? for more about making functions that will have useful asm output. In this case, r0 is the first arg-passing register in the calling convention, and also the return-value register.
I also included another C version using if (a <= 1U) to show that it compiles to the same asm. (1U is an unsigned constant, so C integer promotion rules implicitly convert a to unsigned so the types match for the <= operator. You don't need to explicitly do (unsigned)a <= 1U.)
General case: not a single range
For a case like a==0 || a==3 that isn't a single range-check, you can predicate a 2nd cmp. (Godbolt)
foo:
cmp r0, #3 # sets Z if a was 3
cmpne r0, #0 # leaves Z unmodified if it was already set, else sets it according to a == 0
moveq r0, #1
movne r0, #2
bx lr
You can similarly chain && like a==3 && b==4, or for checks like a >= 3 && a <= 7 you can sub / cmp, using the same unsigned-compare trick as the 0 or 1 range check after sub maps a values into the 0..n range. See the Godbolt link for that.
No that does not work.
cmp r0,#1 is it a one
moveq r0,#1 yes, make it a one again?
movne r0,#2 otherwise make it a 2, what if it was a zero to start, now it is a 2
cmp r0,#0 at this point it is either a 1 or a 2 you forced it so it cannot be zero, what it started off is is now lost.
moveq r0,#1
You have the right concept but need to order things better.
following that line of thinking though
maybe use another register
x = 2;
if(a==0) x = 1;
if(a==1) x = 1;
a = x;
Ponder this
if(a==0) a = 1;
if(a!=1) a = 2;
Or as everyone else is going to say ask the compiler.
because of the or, test OR test, generically they need to be done separately the false condition of the first test does not mean the else condition you have to then do the other test before declaring false. But if true you need to hop over everything and not fall into the second test because that might (in this case will) be false...
As Peter points out you can use unsigned less than or equal and greater than conditions (even though in C it is a signed int, bits is bits).
LS Unsigned lower or same
HI Unsigned higher
Depending the ARM instruction sets is can be:
cmp r0, #1
movls r0, #1
movhi r0, #2
bx lr
or
cmp r0, #1
ite ls
movls r0, #1
movhi r0, #2
bx lr
Am I smarter than you? NO I simply use the compiler to compile the C code.
https://godbolt.org/z/dqxv64Eb9
I understand how the SBC instruction in ARM works.
But, I don't seem to understand how it will be useful, as the intended answer is always less by 1.
Example:
MOV r1, #0x88
MOV r2, #0x44
SUB r3, r1, r2
SBC r4, r1, r2
After this operation, r3 has 0x44 (correct) and r4 has 0x43 (incorrect).
I don't see in which case SBC is a more relevant operation than SUB.
Thanks.
This operation is a substration that adds the carry (PSTATE.C) to the result:
r4 = r1 - r2 - (1-CPSR.C)
CPSR.NZCV has been set by a previous operation that sets flags (For example CMP orADDS).
This type of operation can be useful for large integer additions.
For example, in Aarch32 if you want to calculate a 64-bit addition, you add the 32-bit bottom bits (ADDS) then use ADDC to do the top 32-bit with carry propagation.
I'm new to arm programming and I am trying to understand what the following code does:
.macro set_bit reg_addr bit
ldr r4, =\reg_addr
ldr r5, [r4]
orr r5, #(1 << \bit)
str r5, [r4]
.endm
In particular I am confused about the orr r5,#(1<< \bit) part, I understand that orr stands for logical orr but I am not sure what that means in the given context. I think #(1<<\bit) is seeing if 1 is greater than the given bit, so that will return a true or false statement but I am not sure what the orr command will do.
You are right about the ORR instruction is used for performing logical OR operation. The instruction is used in the following format in the context of this question-
ORR {Register} {Constant}
Now, the constant here is (1 <<\bit) which basically means to left shift 1 by the \bit amount. Here bit is a number between 0-31 which decides which bit needs to be set. The ARM instruction set allows the constant to have such shifting operations as well.
I'm working on writing a program running on Cortex-m3.
At first I wrote an assembly file which executes 'svc'.
svc:
svc 0
bx lr
I decided to use gcc's inline asm, so I wrote it as follows, but the svc function was not inlined.
__attribute__((naked))
int svc(int no, ...)
{
(void)no;
asm("svc 0\n\tbx lr");
}
int f() {
return svc(0,1,2);
}
------------------ generated assembly ------------------
svc:
svc 0
bx lr
f:
mov r0, #0
mov r1, #1
mov r2, #2
b svc
I guess it's not inlined since it is naked, so I dropped the naked attribute and wrote like this.
int svc(int __no, ...)
{
register int no asm("r0") = __no;
register int ret asm("r0");
asm("svc 0" : "=r"(ret) : "r"(no));
return ret;
}
------------------ generated assembly ------------------
svc:
stmfd sp!, {r0, r1, r2, r3}
ldr r0, [sp]
add sp, sp, #16
svc 0
bx lr
f:
mov r0, #0 // missing instructions setting r1 and r2
svc 0
bx lr
Although I don't know why gcc adds some unnecessary stack operations, svc is good. The problem is that svc is not inlined properly, the variadic parameters were dropped.
Is there any svc primitive in gcc? If gcc does not have one, how do I write the right one?
Have a look at the syntax that is used in core_cmFunc.h which is supplied as part of the ARM CMSIS for the Cortex-M family. Here's an example that writes a value to the Priority Mask Register:
__attribute__ ((always_inline)) static inline void __set_PRIMASK(uint32_t priMask)
{
__ASM volatile ("MSR primask, %0"::"r" (priMask));
}
However, creating a variadic function like this sounds difficult.
You can use a macro like this.
#define __svc(sNum) __asm volatile("SVC %0" ::"M" (sNum))
And use it just like any compiler-primitive function, __svc(2);.
Since it is just a macro, it will only generate the provided instruction.