|
Section
06 Part 08 – Homework Results 05 |
|
“Home computers are being called upon to perform many new
functions, including the consumption of homework formerly eaten by the
dog." ~Doug
Larson |
Introduction
The homework
set was a rather tricky one, which required carefully following the code, and
keeping track of register data. But, I
have no doubt that you've done well in at least understanding the flow of it,
even if you didn't get it 100% correct.
Results
|
move.w #$0100,d0 clr.l d1 move.w #$0400,d4 clr.l d2 move.w #$1000,d3 NotReached: addi.b #$10,d2 add.w d0,d1 cmp.w d1,d4 bgt.s NotReached sub.w d2,d1 subi.w #$1000,d3 bpl.s NotReached move.w d1,d0 swap d0 move.w d3,d0 |
d0, d3 and d4 will start with 00000000.
|
move.w #$0100,d0 clr.l d1 move.w #$0400,d4 clr.l d2 move.w #$1000,d3 |
d0 now contains 00000100.
d1 now contains 00000000.
d2 now contains 00000000.
d3 now contains 00001000.
d4 now contains 00000400.
|
addi.b #$10,d2 add.w d0,d1 |
d2 now contains 00000010, and d1
now contains 00000100.
|
cmp.w d1,d4 bgt.s NotReached |
d1 is compared with d4,
0400 - 0100 = 0300. The CCR flags are set accordingly:
|
C |
V |
Z |
N |
X |
|
0 |
0 |
0 |
0 |
- |
C is
cleared (No carrying outside the word range), V is cleared (positive – positive = positive), Z is cleared (Not zero), N is cleared (Not negative), X
is ignored (Not changed by CMP).
The
instruction BGT will branch if the destination operand is greater than the source operand (CCR: so long as Z is cleared, and N and V are both cleared or
both set). Since 0400 is greater than 0100 (Z, N and V are all cleared),
the 68k will branch to the lable "NotReached:", and continue there.
Once
again:
|
addi.b #$10,d2 add.w d0,d1 |
d2 now contains 00000020, and d1
now contains 00000200.
|
cmp.w d1,d4 bgt.s NotReached |
d1 is compared with d4,
0400 - 0200 = 0200. Since 0400 is greater than 0200, branch to "NotReached:".
|
addi.b #$10,d2 add.w d0,d1 |
d2 now contains 00000030, and d1
now contains 00000300.
|
cmp.w d1,d4 bgt.s NotReached |
d1 is compared with d4,
0400 - 0300 = 0100. Since 0400 is greater than 0300, branch to "NotReached:".
|
addi.b #$10,d2 add.w d0,d1 |
d2 now contains 00000040, and d1
now contains 00000400.
|
cmp.w d1,d4 bgt.s NotReached |
d1 is compared with d4,
0400 - 0400 = 0000. Since 0400 is NOT greater than 0400 (Z flag is now set because
the result is zero), the branch will not occur, the 68k will now continue:
|
sub.w d2,d1 |
0400 - 0040 = 03C0. d1 now contains 000003C0.
|
subi.w #$1000,d3 bpl.s NotReached |
1000 - 1000 = 0000. d3 now contains 00000000.
The
BPL flag will branch because the result is still positive (The N flag is clear). branch to "NotReached:".
|
addi.b #$10,d2 add.w d0,d1 |
d2 now contains 00000050, and d1
now contains 000004C0.
|
cmp.w d1,d4 bgt.s NotReached |
d1 is compared with d4,
0400 - 04C0 = FF40. 0400 is NOT greater than 04C0 (N flag is now set because
the result is negative, the V flag is cleared because the answer is perfectly
valid), because N and V are not the same, the branch will NOT occur, the 68k
will now continue:
|
sub.w d2,d1 |
04C0
- 0050 = 0470. d1 now contains 00000470.
|
subi.w #$1000,d3 bpl.s NotReached |
0000 - 1000 = F000. d3 now contains 0000F000.
The
BPL flag will NOT branch because the result is negative (The N flag is set), so the 68k continues:
|
move.w d1,d0 |
0470 is
copied from d1 to d0. d0 now contains 00000470.
|
swap d0 |
d0 now contains 04700000.
|
move.w d3,d0 |
F000
is copied from d3 to d0. d0 now contains 0470F000.
The
result of d0 is 0470F000. And as you can
see, the 68k looped several times there while making the calculations
necessary. This shows you not only how
conditional branches work, but how they can be used to make certain specific
calculations using the same instructions several times over.