CPE215 Lecture 31

1
CPE-215 Lecture 31

• Motorola Code

2
Line Fields

• Each line in MC68000 is divided into four fields
• Label
• Opcode Mnemonic
• Operand(s)
• Comment
• Label and Comment are optional.

3
MC68000 Syntax

• Directives
• XREF directive
• Assembly Language Instructions
• Data Section
• DATA directive
• Define Constant directives
• Define Storage directives
• End Directive

4
XREF Directives

• XREF permits access to external subroutines, such
  as those for keyboard input, line termination,
  and display output.
• Every assembler has a number of these, and they
  are linked in when the program is assembled.

5
Input/Output Routines

• Each assembler has routines to get inputs from
  the keyboard and display outputs on the screen.
• These are linked into the program in response to
  the XREF directive.
• DECIN, DECIN__LONG
• DECOUT, DECOUT__LONG
• HEXIN, HEXIN__LONG
• HEXOUT, HEXOUT__LONG
• NEWLINE
• STRIN
• STROUT

6
STRIN

• You need a buffer
• bufname DS.B 80
• The buffer address is loaded into A0
• LEA bufname,A0
• The buffer size is loaded into D0
• MOVE.W 80,D0
• Then you jump to the subroutine
• JSR strin

7
Where Stuff Goes

• Input subroutines store the input from the
  keyboard in register D0.
• Output subroutines store whatever is in D0 to the
  display.
• In the case of strings, A0 holds the address of
  the string or buffer, and D0 holds the length of
  the string or size of the buffer.

8
STROUT

• You need a string
• msg DC.B This string has 29 characters
• The string address is loaded into A0
• LEA msg,A0
• The string size is loaded into D0
• MOVE.W 29,D0
• Then you jump to the subroutine
• JSR strout

9
DATA Directive

• This directive is used to define the beginning of
  the data section.

10
Data Directives

• Define Constant
• DC.ltsizegt ltconstantsgt
• Can be Byte, Word, Long word
• Define Storage
• DS.ltsizegt ltcountgt
• Can be Byte, Word, Long word
• Not initialized
• Define Constant Block
• DCB.ltsizegt ltcountgt,ltvaluegt
• Can be Byte, Word, Long word
• All of these can be used to create arrays

11
Sample Program 1 -- Pascal

• program example (output)
• var
• i, j, k integer
• begin
• i 75
• j 4
• k i j 6
• writeln (k)
• end.

12
Sample Program 1 -- C

• include ltstdio.hgt
• void main (void)
• int i 75
• int j 4
• int k
• k i j 6
• printf (d\n, k)

13
MC68000 Code, Sample 1

• xref decout, newline, stop
• program instruction section
• start move.w I,d0
• add.w j,d0
• subi.w 6,d0
• move.w d0,k
• jsr decout
• jsr newline
• jsr stop
• data declaration section
• data
• i dc.w 75
• j dc.w 4
• k ds.w 1
• end

14
MC68000 Code, Sample 2

• This program computes 2n, where n is input.
• Number must be in the range of 0 to 14.
• The table entry for n is printed out.
• NOTE This sample does not include any prompts
  for input, so the code could be improved vastly
  for user-friendliness.

15
Code for Sample 2

• xref decout, decin, newline, stop
• start lea p2table,a0
• jsr decin
• bmi out
• cmpi.w 14,d0
• bgt out
• asl.w 1,d0
• move.w (0,a0,d0.w),d0
• jsr decout
• jsr newline
• out jsr stop
• data
• p2table dc.w 1,2,4,8,16,32,64,128,256,512,1024
• dc.w 2048,4096,8192,16384
• end

16
Sample 3, Subroutine

• This program computes Y X2 X using a
  subroutine.
• Since D1 is used as a scratch register by the
  subroutine, it must be pushed and popped within
  the subroutine.

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Sample 3, Main Program

• XREF DECOUT,NEWLINE,STOP
• START MOVEQ 6,D1 Loop 7 times
• MOVEQ 1,D2
• LOOP MOVE.W D2,D0
• JSR COMPUTE
• JSR DECOUT
• JSR NEWLINE
• ADDQ.W 1,D2
• DBRA D1,LOOP
• JSR STOP
• COMPUTE
• END

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Sample 3, Subroutine

• COMPUTE MOVE.L D1,-(SP)
• Push D1 on the stack
• MOVE.W D0,D1
• MULU D0,D1
• ADD.W D1,D0
• MOVE.L (SP),D1 Pop D1
• RTS

19
Saving More Stuff

• Save
• MOVE.L A0,-(SP)
• MOVE.L D2,-(SP)
• MOVE.L D1,-(SP) MOVEA.L ?
• or
• MOVEM.L D1-D2/A0,-(SP)
• Restore
• MOVE.L (SP),D1
• MOVE.L (SP),D2
• MOVEA.L (SP),A0
• or
• MOVEM.L (SP),D1-D2/A0