1. ECE 3430 – Intro to Microcomputer Systems
ECE 3430 – Introduction to Microcomputer Systems University of Colorado at Colorado Springs
Lecture #10
Agenda Today 1) Jump Instructions/Conditional Execution 2) Status register flags: N,Z,V,C 3) Compare Instructions
Lecture #10
ECE 3430 – Intro to Microcomputer Systems
Fall 2014

2. Jump Instructions
We need a way to alter the flow of program execution when specific conditions exist.
This allows our programs to dynamically adjust to input.
The Status Register provides the status of the CPU.
V = Overflow
N = Negative Result Z = Zero Result C = Carry
START
<op>
<loop op>
<cond> x == y? NO
YES
Only these flags affect behavior of conditional jump instructions
<op>
END
Lecture #10
ECE 3430 – Intro to Microcomputer Systems
Fall 2014

3. ECE 3430 – Intro to Microcomputer Systems
Jump Instructions
SR[8,2:0] = {V,N,Z,C} - These flags are altered upon execution of an instruction (check instruction set to see which flags). - We can use a conditional jump instruction depending on these flags to set the new value
of the PC. - A jump occurs when the PC is set to different place in the code (besides the next
sequential instruction). - Jumps in the MSP430 always use 10-bit relative addressing. This gives a range of –512 to
+511 words relative to where the PC is starting. In other words, the PC can be
changed to a value within –512 to +511 words of the current PC location. - Labels provide an easy way to mark the destination of a jump. This way, the assembler can calculate the jump instruction’s operand value for you!
- Unconditional Jump – “jmp” is an instruction that will always jump when executed
regardless of what bits are set in the SR. “jmp” is the only
unconditional jump instructions in MSP430 instruction set.
- Conditional Jump – When executed, it will check the SR and evaluate a logical condition If the jump condition is true, it will set the PC to the new address location. If the jump condition is false, it will increment the PC by 2
as in normal operation. All jump instructions other than “jmp” are
conditional.
Lecture #10
ECE 3430 – Intro to Microcomputer Systems
Fall 2014

4. ECE 3430 – Intro to Microcomputer Systems
Conditional Jumps
Carry Flag Jumps
jnc = jump if carry clear, C=0
jc = jump if carry set, C=1
jhs = jump if higher or same, C=1
jlo = jump if lower, C= ORG 0xC mov.b #0xF0,R4 Loop: add.b #1,R4 jnc Loop ; C = 0  jump ; C = 1  no jump mov.b R4,0x END
START
alias
alias
R4 = 0xF0
R4 = R4 +1
SUM > 255? NO
YES
M(0x0200) = R4
END
Lecture #10
ECE 3430 – Intro to Microcomputer Systems
Fall 2014

5. ECE 3430 – Intro to Microcomputer Systems
Conditional Jumps
Carry Flag Jumps How many times did this loop execute? Loop1  R4=0xF1 Loop2  R4=0xF2 Loop3  R4=0xF3 : : Loop15  R4=0xFF Loop16  R4=0x00, C=1, No Jump ANSWER = 16 times
START
R4 = 0xF0
R4 = R4 +1
SUM > 255? NO
YES
M(0x0200) = R4
END
Lecture #10
ECE 3430 – Intro to Microcomputer Systems
Fall 2014

6. ECE 3430 – Intro to Microcomputer Systems
Conditional Jumps
Carry Flag Jumps – Add M(0x0200) and M(0x0201), 8-bit - If the result is =< 255, set P1.5 (clear all other bits in port 1) and END - If the result is > 255, set P1.4 (clear all other bits in port 1)
and END ORG 0xC000 MAIN: mov.b 0x0200, R4 add.b 0x0201, R4 jc Error ; C = 1 = branch ; C = 0 = no branch bis.b # b,&P1OUT jmp Finish Error: bis.b # b,&P1OUT Finish: jmp Finish
START
M(0x0200) + M(0x0201)
SUM > 255?
YES
P1.4 = 1 NO
P1.5 = 1
END
Lecture #10
ECE 3430 – Intro to Microcomputer Systems
Fall 2014

7. ECE 3430 – Intro to Microcomputer Systems
Conditional Jumps
Carry Flag Branches – What does the machine code look like?
Addr: IC: Operands:
C FE mov.b 0x0200,R4
C FB add.b 0x0201,R4
C008 2C jc Error
C00A D0F bis.b # b,&P1OUT
C00E 3C jmp Finish
C010 D0F Error: bis.b # b,&P1OUT
C014 3FFF Finish: jmp Finish
Lecture #10
ECE 3430 – Intro to Microcomputer Systems
Fall 2014

8. ECE 3430 – Intro to Microcomputer Systems
Conditional Jumps
How long does it take to execute if the branch IS
taken?
How long does it take to execute if the branch IS NOT
What is the average execution time?
-> In the previous example, the time is the same
whether the branch is taken or not.
Lecture #10
ECE 3430 – Intro to Microcomputer Systems
Fall 2014

9. ECE 3430 – Intro to Microcomputer Systems
Conditional Jumps
Zero Flag Branches jeq/jz = jump if equal to zero, Z=1 jne/jnz = jump if NOT equal to zero, Z=0 Ex) loop 10 times ORG 0xC mov.b #10,R4 Loop: dec.b R4 jne Loop ; Z = 0 = jump ; Z = 1 = no jump END
START
count = 10
count = count - 1
count = 0? NO
YES
END
Lecture #10
ECE 3430 – Intro to Microcomputer Systems
Fall 2014

10. ECE 3430 – Intro to Microcomputer Systems
Conditional Jumps
Zero Flag Branches How many loops? Loop1  A=9
Loop2  A=8
: Loop10  A=0 (Z=1, jump NOT taken) How long to execute? Loop  dec.b R4  2 cycles jne  2 cycles (4 cycles)(10 loops) = 40 cycles mov.b #10,R4  2 cycles TOTAL = (42 cycles)(<period of MCLK>) = <execution time>
START
count = 10
count = count - 1
count = 0? NO
YES
END
Lecture #10
ECE 3430 – Intro to Microcomputer Systems
Fall 2014

11. ECE 3430 – Intro to Microcomputer Systems
Conditional Jumps
Negative Flag Branches jn = jump if negative, N=1 (jump if minus) Can be used during subtraction to indicate the value was negative. Ex) M(0x0020) – M(0x0021) if negative or zero, END if positive, set P1.4 (clear all other bits on port 1) ORG 0xC000
mov.b 0x0200, R4 sub.b 0x0201, R4 jn DONE mov.b # b, &P1OUT DONE: jmp $ ;  $ is a quick way to jump back to same line (trap PC)
END
Lecture #10
ECE 3430 – Intro to Microcomputer Systems
Fall 2014

12. ECE 3430 – Intro to Microcomputer Systems
Conditional Jumps
Two’s Complement Overflow Flag Jumps jge = jump if greater than or equal to ; N  V = 0 jl = jump if less than ; N  V = 1
None that test only the V flag!
Notice that a logic equation is evaluated for correctness!
Lecture #10
ECE 3430 – Intro to Microcomputer Systems
Fall 2014

13. Signed vs. Unsigned Jump Instructions (Summary)
Unsigned branch instructions: Equation:
jhi = jump if higher than
jhs = jump if higher than or the same  same as jc C ?= 1
jlo = jump if lower than  same as jnc C ?= 0
jls = jump if lower than or the same
Signed branch instructions:
jge = jump if greater than or equal (*) N xor V ?= 0
jgt = jump if greater than
jle = jump if less than or equal
jl = jump if less than (*) N xor V ?= 1
jn = jump if negative flag is set (jump if minus) N ?= 1
Neither:
jnc = jump if carry flag is clear  same as jlo C ?= 0
jc = jump if carry flag is set  same as jhs C ?= 1
jeq/jz = jump if equal (zero flag is set) Z ?= 1
jne/jnz = jump if not equal (zero flag is clear) Z ?= 0
jmp = jump always (unconditional jump) ----
= DOES NOT EXIST
= UNCONDITIONAL
* = checks N and V flags
Lecture #10
ECE 3430 – Intro to Microcomputer Systems
Fall 2014

14. ECE 3430 – Intro to Microcomputer Systems
Conditional Jumps
Compare Instructions
Jump instructions do not modify SR bits. They simply read the current state of the
N,V,C,Z flags. At times we want to alter the SR but not change the contents of the registers. By
altering the state of the SR bits, we can control what value the jump condition is relative to
--rather than always zero. Compare instructions allow us to do this: cmp <src>,<dst> = compare two values (destination not written), <dst> - <src>
tst <dst> = test for zero or minus condition
Typically, the above compare/test instructions should immediately precede a
jump instruction!
Ex: Consider ‘a’ and ‘b’ to both be unsigned. The expression “if (a >= b) then <label>”
is evaluated:
cmp b, a or cmp a, b NOTE: In MSP430, “jls” does
jhs <label> jls <label> not exist!
Lecture #10
ECE 3430 – Intro to Microcomputer Systems
Fall 2014

15. Using Compare Instructions to Influence Jump Instruction Behavior
ORG 0xC000
Start: mov.b #120,R4
cmp.b 0x0200,R4
jl Set4
Clr4: mov.b &P1OUT,R4
and.b # b,R4
jmp Rejoin
Set4: mov.b &P1OUT,R4
bis.b # b,R4
Rejoin: mov.b R4,&P1OUT
START
R4 = 120
R4 < M(0x0200)?
YES
P1.4 = 1 NO
P1.4 = 0
What could be done to eliminate the redundant
“mov.b &P1OUT,R4” instruction?
How many other ways could this code be written?
END
Lecture #10
ECE 3430 – Intro to Microcomputer Systems
Fall 2014

16. Using Compare Instructions to Influence Jump Instruction Behavior
JGE (signed) takes branch if “N  V = 0” evaluates as true.
JHS (unsigned) takes branch if “C = 1” evaluates as true.
Assume M(0x0200) = 128
N Z V C
mov.b 0x0200,R
cmp.b #120,R
jge Set4 0  1 = 0 1 = 0 no jump
jhs Set4 1 = = 1 jump
= negative – positive = positive (overflow, V = 1).
Subtracted smaller number from larger (no borrow, C = 1).
Result not negative (N = 0).
Two values were not the same (Z = 0).
Lecture #10
ECE 3430 – Intro to Microcomputer Systems
Fall 2014

17. Using Compare Instructions to Influence Jump Instruction Behavior
JGE (signed) takes branch if “N  V = 0” evaluates as true.
JHS (unsigned) takes branch if “C = 1” evaluates as true.
Assume M(0x0200) = 100
N Z V C
mov.b 0x0200,R
cmp.b #120,R
jge Set4 1  0 = 0 1 = 0 no jump
jhs Set4 0 = = 1 no jump
= positive – positive = negative (no overflow, V = 0).
Subtracted larger number from smaller (borrow, C = 0).
Result is negative (N = 1).
Two values were not the same (Z = 0).
Lecture #10
ECE 3430 – Intro to Microcomputer Systems
Fall 2014