1. Logical Operations bitwise logical operations AND, OR, EOR, NOT
“mask” = setting specific bits to zero
e.g. mask high four bits in NUM NUM DC.B $4C
for bit operations, could mask out all the bits you are not interested in testing

2. Utilization of Bits to Store Data - the case of 8 doors; 1 indicates door is open
DOORS DC.B $05 or DOORS DC.B % Are the even numbered doors closed?
DOOR1 DC.B 1 DOOR2 DC.B 0 DOOR3 DC.B 1 DOOR4 DC.B 0 … DOOR8 DC.B 0 D1 D3 D5 D7 D2 D4 D6 D8
hallway

3. Shift Operations directions: left, right
types: logical, arithmetic, circular
length: 1 bit, > 1 bit
flags: Z, N, C, sometimes X

4. Shift Operations Right: logical ≠ arithmetic
Left: logical ≡ arithmetic C
lsr X
asl lsl C C
asr X X
rol C C
ror
roxl X X C C
roxr

5. Utilization of Bits to Store Data - the case of 8 doors; 1 indicates door is open
DOORS DC.B $05 or DOORS DC.B % How many doors are open?
DOOR1 DC.B 1 DOOR2 DC.B 0 DOOR3 DC.B 1 DOOR4 DC.B 0 … DOOR8 DC.B 0

6. Bit Operations instructions operate on one bit in Dn or memory
bit numbering:
Dn =
(long) …
memory
(byte)

7. Bit Operations e.g. NUM1 DC.B $A2 D0=$0000F8AB
Bit Test BTST #$F,D0 z ← ~(<bit number>) of destination
Bit Set = test a bit then set BSET #3,NUM z ← ~(<bit number>) of destination <bit number> of destination ← 1
Bit Clear = test a bit then clear BCLR #8,D0 z ← ~(<bit number>) of destination <bit number> of destination ← 0
Bit Change = test a bit then toggle BCHG #1,NUM z ← ~(<bit number>) of destination <bit no> of dest ← ~<bit no> of dest
* source operand could be Dn (contains bit position)

8. Utilization of Bits to Store Data - the case of 8 doors; 1 indicates door is open
DOORS DC.B $05 or DOORS DC.B % Is door 3 open?
DOOR1 DC.B 1 DOOR2 DC.B 0 DOOR3 DC.B 1 DOOR4 DC.B 0 … DOOR8 DC.B 0

9. Set according to condition
used primarily to set/clear flags
Scc = set a byte according to condition cc
if (condition true) then destination ← $FF ; set to “true” else destination ← $00 ; set to “false” endif
e.g. SEQ Flag … Flag DC.B $00

10. Note: Aliases are set elsewhere. We will see this code again later.
/***************************************************************/
/* Purpose..: Compute result = left << right */
/* Input....: pointer to result */
/* Input....: left operand */
/* Input....: shift count */
/* Return...: pointer to result */
extern asm CInt64* __rt_shl64(CInt64 *result,CInt64 left,short count) {
move.l LEFT_LO,d0
move.l LEFT_HI,d1
move.w SHIFT_COUNT,d2
and.w #0x003F,d2
bra.s l1
l0: add.l d0,d0
addx.l d1,d1
l1: dbf d2,l0
move.l RESULT,a0
move.l d0,RESULT_LO
move.l d1,RESULT_HI
rts }
Note: Aliases are set elsewhere. We will see this code again later.

11. /***************************************************************/
/* Purpose..: Compute result = left >> right (signed) */
/* Input....: pointer to result */
/* Input....: left operand */
/* Input....: shift count */
/* Return...: pointer to result */
extern asm CInt64* __rt_shrs64(CInt64 *result,CInt64 left,short count) {
move.l LEFT_LO,d0
move.l LEFT_HI,d1
move.w SHIFT_COUNT,d2
and.w #0x003F,d2
bra.s l1
l0: lsr.l #1,d0
asr.l #1,d1
bcc.s l1
bset #31,d0
l1: dbf d2,l0
move.l RESULT,a0
move.l d0,RESULT_LO
move.l d1,RESULT_HI
rts }

12. /***************************************************************/
/* Purpose..: Compute result = __rol(left,right) */
/* Input....: pointer to result */
/* Input....: left operand */
/* Input....: shift count */
/* Return...: pointer to result */
extern asm CInt64* __rt_rotl64(CInt64 *result,CInt64 left,short count)
{ move.l LEFT_LO,d0
move.l LEFT_HI,d1
move.w SHIFT_COUNT,d2
and.w #0x003F,d2
bra.s l1
l0: add.l d0,d0
addx.l d1,d1
bcc.s l1
addq.w #1,d0
l1: dbf d2,l0
move.l RESULT,a0
move.l d0,RESULT_LO
move.l d1,RESULT_HI
rts }

13. Reading, Expectations Reading: Expectations:
M68000 Assembly Language [pdf, 92p; N. Znotinas]
review operation of instructions covered in presentation
examples were taken from the PalmOS 64 bit arithmetic library, LongLong68K.c
Expectations:
you can explain the operation of and the differences between the various shifts and rotates
you can read/write code that uses all of the above instructions, eg. the PalmOS 64 bit arithmetic library