1. Write a program to calculate x**y, given x and y.
(revisited)
inputs: x (w – because of the multiply instruction) y (b – it is a counter) output: ans (l – because of the multiply instruction)
org $400 main move.w x,D0 get base move.w x,D2 and make copy move.b y,D1 get exponent beq skip if zero, skip calc loop mulu D0,D2 do product sub.b #1,D1 decrement counter bne loop skip move.l D2,ans save x**y
stop #$2700 We have some problems:
x dc.w 5 y dc.b 3 ans ds.l 1
end main

2. Valid flags for signed arithmetic
e.g. A – B
if N=0 A – B 0
if N=1 A – B 0
if N=V A B
if N≠V A B
if N=V and Z=0 A B
if N≠V or Z=1 A B
V = 1 → result incorrect → signed overflow 0 → result correct
Z = 1 → result is zero
0 → result is not zero
N = 1 → result is negative
0 → result is positive (includes zero)

3. Decision Structures … if-then
in assembly language
in pseudo-code: …
if condition then … code A
end if … code B
“if condition then” decomposes into a series of operations 1) test condition → sets flags 2) branch on the flags

4. Decision Structures … testing conditions
to “test” condition:
arithmetic
logical
shifts, rotates
data movement
compare, test
CMP <ea>,Dn
[destination] - [source]
e.g. move.b #5,D0
cmp.b #6,D0
TST <ea>
compare [destination] to 0 ≡ [destination] – 0
e.g. move.b #-1,D0
tst.b D0

5. Conditional Branch …Bcc
M68000 Assembly Language [p16, N. Znotinas]
conditional branches, an alternate view:
BHI | BLS Br high |Br low or same
BCC | BCS Br C clear |Br C set
BHS*| BLO* Br high/same |Br low
BNE | BEQ Br not equal |Br equal
BVC | BVS Br V clear |Br V set
BPL | BMI Br plus |Br minus
BGE | BLT Br ≥ |Br <
BGT | BLE Br > |Br ≤
* depends on assembler if synonym accepted

6. Decision Structures … if-then-else
in assembly language
in pseudo-code: … if condition then … code A else … code B end if … code C
Assembly code logic should flow
from top to bottom like a waterfall.

7. Testing for out-of-range data/results
perform arithmetic input operation data ↓
result/data in range? → error
continue processing
input data and calculated data may be out-of-range, i.e. too large to store correctly
you are responsible for testing for out-of-range conditions
you would not test …

8. Testing for out-of-range results
e.g. ** technique 1 …
ADD.W D0,D1
B Ans_OK
… code to put … out err msg
JMP END_M
Ans_OK … next code
… section
END_M
e.g. ** technique 2 …
ADD.W D0,D1
B V_err
… next code
… section
JMP END_M
V_err … code to put … out err msg …
END_M

9. Loops … conditional loops
in assembly language
in pseudo-code: … while condition do
… code A
end while
… code B

10. e.g. If a 16 bit number is negative, generate the absolute value of the number. Store the result in the number’s original memory location.

11. e.g. Find the larger of two 32 bit numbers.

12. M68000 Assembly Language [pdf, 92p; N. Znotinas]
Reading:
M68000 Assembly Language [pdf, 92p; N. Znotinas]
Look at the Bcc command and relate the use of the condition codes in the table to the relationships we developed in the last two lectures. The relationships are the same but written in different formats.
Look at the unconditional branch commands: BRA, JMP
review the operation of the various binary add (ADD, ADDA, ADDI, ADDQ) and subtract (SUB, SUBA, SUBI, SUBQ) instructions
review the operation of the test (TST) instruction and the various forms of the compare (CMP, CMPA, CMPI) instruction
review the operation of the MULS|MULU (multiply) and DIVS|DIVU (divide) instructions
Expectations:
starting with assignment 4, we will expect arithmetic results to be checked for out-of-range results when appropriate
you can write any decision structure (if, if-then-else, if-elseif, case) in assembly language
you can write a simple looping structure in assembly language
you can select the correct conditional branch for any given signed or unsigned test