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Logical Operations

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.

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Logical Operations

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  1. Logical Operations • bitwise logical operations • AND, OR, EOR, NOT • “mask” = setting specific bits to zero • e.g. mask high four bits in NUMNUM 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 $05or DOORS DC.B %00000101Are the even numbered doors closed? DOOR1 DC.B 1DOOR2 DC.B 0DOOR3 DC.B 1DOOR4 DC.B 0…DOOR8 DC.B 0 D1D3 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. Left: logical ≡ arithmetic Right: logical ≠ arithmetic Shift Operations 0 lsr C X asllsl C 0 C asr X X rol C ror C X X roxr C roxl C

  5. Utilization of Bits to Store Data- the case of 8 doors; 1 indicates door is open DOORS DC.B $05or DOORS DC.B %00000101How many doors are open? DOOR1 DC.B 1DOOR2 DC.B 0DOOR3 DC.B 1DOOR4 DC.B 0…DOOR8 DC.B 0

  6. Bit Operations • instructions operate on one bit in Dn or memory • bit numbering: Dn = (long) 31 … 1 0 memory 7 6 5 4 3 2 1 0 (byte)

  7. e.g. NUM1 DC.B $A2 D0=$0000F8AB Bit Operations • Bit Test BTST #$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 $05or DOORS DC.B %00000101Is door 3 open? DOOR1 DC.B 1DOOR2 DC.B 0DOOR3 DC.B 1DOOR4 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. /***************************************************************/ /* 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: • 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

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