680XX Program Examples

1
680XX Program Examples

• Outline
• Binary to BCD
• Matrix Addition
• Ones Count
• String Compare
• Sector Map
• Raster Graphics
• Subroutine Calls
• Goal
• Understand instruction usage
• Reading
• Microprocessor Systems Design, Clements, Ch. 2-3

2
Binary to BCD Conversion

• D0.B contains binary value
• D0.W contains 3-digit BCD result
• CLR.L D1 Clear D1 for 32-bit dividend
• MOVE.B D0,D1 Copy source to D1
• DIVU.W 100,D1 Get 100s digit in D1(015)
• MOVE.W D1,D0 Save digit in D0(03)
• SWAP D1 Move remainder to D1(015)
• AND.L FFFF,D1 Clear MSW of D1
• DIVU 10,D1 Get 10s digit in D1(015)
• LSL.W 4,D0 Shift 100s digit one place
• OR.W D1,D0 Insert 10s digit in D0
• LSL.W 4,D0 Shift digits one place
• SWAP D1 Move remainder to D1(015)
• OR.W D1,D0 Insert 1s digit into LSN

3
Conversion Example

• 11001100 (204) to 0010 0000 0100
• CLR.L D1 D0 000000CC, D1 00000000
• MOVE.B D0,D1 D0 000000CC, D1 000000CC
• DIVU.W 100,D1 D0 000000CC, D1 00040002
• MOVE.W D1,D0 D0 00000002, D1 00040002
• SWAP D1 D0 00000002, D1 00020004
• AND.L FFFF,D1 D0 00000002, D1 00000004
• DIVU 10,D1 D0 00000002, D1 00040000
• LSL.W 4,D0 D0 00000020, D1 00040000
• OR.W D1,D0 D0 00000020, D1 00040000
• LSL.W 4,D0 D0 00000200, D1 00040000
• SWAP D1 D0 00000200, D1 00000004
• OR.W D1,D0 D0 00000204, D1 00000004

4
Matrix Addition

• C A B, A, B, C are m x n matrices
• store matrix by rows - row order
• a1,1 stored at A, ai,j stored at A(i-1)nj-1
• MOVEA.L A,A0 A0 is base of matrix A
• MOVEA.L B,A1 A1 is base of matrix B
• MOVEA.L C,A2 A2 is base of matrix C
• CLR.W D2 Clear element offset
• MOVE.W m,D0 D0 is row counter
• L2 MOVE.W n,D1 D1 is column counter
• L1 MOVE.B (A0,D2.W),D6 Get element from A
• ADD.B (A1,D2.W),D6 Add element from B
• MOVE.B D6,(A2,D2.W) Store sum in C
• ADDQ.W 1,D2 Increment element pointer
• SUB.W 1,D1 Repeat for n columns
• BNE L1
• SUB.W 1,D0 Repeat for m rows
• BNE L2

5
Ones Count

• Subroutine to count number of 1s in byte
• D0.B - input/output register
• D1 - ones counter (not modified)
• D2 - pointer to bit of D0 to be tested (not
  modified)
• ONE_CNT MOVEM.L D1-D2,-(A7) Save D1 and D2
• CLR.B D1 Clear 1s counter
• MOVEQ 7,D2 D2 points to MSB
• NXT_BIT BTST D2,D0 Test D2th bit of D0
• BEQ.S LP_TST Do nothing if 0
• ADDQ.B 1,D1 Else incr 1s cnt
• LP_TST SUBQ.B 1,D2 Decr bit pointer
• BGE NXT_BIT Repeat until done
• MOVE.B D1,D0 Put count in D0
• MOVEM.L (A7),D1-D2 Restore D1 and D2
• RTS Return

6
Ones Count

• Usage
• MOVE.B ltdatagt,D0 Avoid by having data in D0
• JSR ONE_CNT Jump to subroutine
• MOVE.B D0,ltdestgt Avoid by using result in D0
• note that D1 and D2 are saved and restored by
  subroutine
• to/from stack
• Alternative
• use BSR, let assembler compute offset
• use global register allocation to avoid stack
  save/restore
• use register windows to avoid stack save/restore
  - SPARC

7
String Compare

• Compare strings S1 and S2 of length n
• return 1 if S1 gt S2, 0 if S1 S2, -1 if S1 lt S2
• A0 points to S1, A1 points to S2, n in D0, result
  in D0
• SUBQ 1,D0 D0 is byte counter
• L1 CMPM.B (A0),(A1) Compare characters
• BLT LT S1 lt S2
• BGT GT S1 gt S2
• DBRA D0,L1 Repeat until done
• CLR.L D0 D0 0
• RTS Return
• LT MOVEQ -1,D0 D0 -1
• RTS Return
• GT MOVEQ 1,D0 D0 1
• RTS Return
• DBRA DBF - condition always false, so loop

8
Sector Map

• Disk of 2048 256-byte sectors (512KB)
• sector map - vector of 2048 bits (64 longwords)
• 1 bit per sector
• bit is 1 if sector is free, 0 if sector is used
• find first free sector and claim it
• CLR.L D0 Initial bit offset D0 0
• LEA MAP,A0 A0 points to sector bitmap
• MOVE.W 63,D7 Up to 64 fields to test
• L1 BFFFO (A0)D032,D0 If free sector found, Z0
• and D0 offset from A0
• DBNE D7,L1 Decr D7 until Z0 or end
• BEQ FULL Disk full
• BFCLR (A0)D01 Claim sector, D0 sector

9
Raster Graphics
0,0
Store display in row order
x1,y1
x2,y2
1023,767

• Copy 15x15 block from (x1,y1) to (x2,y2)
• e.g. bitmapped character set

10
Raster Graphics

• A - origin of bitmap
• D0 - x1, D1 - y1, D2 - x2, D3 - y2
• MV LEA A,A0 A0 base address of bitmap
• MULU.L 128,D1 D1 src row offset
• MULU.L 128,D3 D3 dest row offset
• MOVEQ 14,D4 15 lines to move
• L1 BFEXTU (A0,D1.L)D0.L15,D5 Copy line to D5
• BFINS D5,(A0,D3.L)D2.L15 Copy into image
• LEA 128(A0),A0 Update pointer by a line
• DBRA D4,L1 Repeat until all lines moved
• RTS Return

11
Subroutine Calls

• Call with BSR, JSR, return with RTS, RTD, RTR
• RTD if deallocating stack frame, RTR if restoring
  CCR
• Pass parameters by value
• put data in data registers
• save previous data on stack if necessary
• example
• MOVE.L D0,-(SP)
• MOVE.L lteagt,D0
• BSR SUBR
• Pass parameters by reference
• put address in address registers
• save previous addresses on stack if necessary
• example
• MOVE.L A0,-(SP)
• LEA 002000,A0
• BSR SUBR

12
Subroutine Calls

• Pass via stack
• data and references
• example
• PEA TEXT_ST Push text starting addr
• PEA TEXT_END Push text ending addr
• PEA STR_ST Push string starting addr
• PEA STR_END Push string ending addr
• BSR STR_MT Call subroutine
• LEA 16(SP),SP Pop stack
• ...
• STR_MT LEA 4(SP),A0 Parameter ptr in A0
• MOVEM.L A3-A6,-(SP) Save registers
• MOVEM.L (A0),A3-A6 Get params off stack
• ...
• MOVEM.L (SP),A3-A6 Restore registers
• RTS Return

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Subroutine Calls
Stack
Return Address
SP after BSR
STR_END ptr
SP before BSR
A0 after LEA 4(SP),A0
Memory
STR_ST ptr
A0 after MOVEM
TEXT_END ptr
TEXT_ST ptr
String
A3
SP initial value
A4
...
A5
A6
Text