Introduction to M68000 Microprocessor II. Physics116B, 3/4/05 D. Pellett References: Motorola literature, Wilkinson, Ford and Topp, Horowitz and Hill

Transcription

1 Introduction to M68000 Microprocessor II Physics116B, 3/4/05 D. Pellett References: Motorola literature, Wilkinson, Ford and Topp, Horowitz and Hill

2 Outline Today: simple programs to show how things work Look at binary representations of typical instructions Look at simple constructs: loops, subroutines, use of stack Branch instructions; indirect addressing Next: do simple task replacing random logic hardware Learn to connect to external device via SCSI bus hack Generate waveform from data array As time permits Use DAC to make successive approximation ADC Consider how to wire ADC chip to bus

3 Assembly Language Introduction Code structure and directives Code representation in computer Data transfer, integer arithmetic, logical Instructions External I/O subroutines for keyboard, screen, file access Branch Instructions for loops Array (and other) access with indirect addressing Special conventions used by Macintosh How to make and use local subroutines Use of run-time stack for subroutine communication

4 Simple Program Example: Read and Store One ASCII Character Note use of directives, code and MAS subroutines Note use of labels, comments, byte addressing for move.b If you input a, $61 will be stored in the location msg

5 Simple Program Yields Complex Code Program is assembled, loaded into memory Statement labels are assigned to locations in memory: For example, start = $ Run with debugger On entry to the program, PC=$ Memory contents as follows (note numbering in bytes):

6 Decode First Instruction Connection between assembly language and computer code First instruction start: jsr getchar becomes $ : $4eba $045c Look at JSR details: Indeed, $4e is first byte of JSR code... What about the lower byte? Figures from M68000 Microprocessors User s Manual 9th Ed., 1993 Motorola Inc.

7 Find Effective Address of JSR Entry Point Lower byte = $ba Second byte is $ba = % Spaces added to show effective address (EA) pattern Mode = %111, Register = %010 Table above says this EA is of form (d16, PC) Next 16 bit word ($045c) is the displacement relative to PC Figures from M68000 Microprocessors User s Manual 9th Ed., 1993 Motorola Inc.

8 Where is That Subroutine, Anyway? (d16,pc) means the address of the entry to the subroutine is the sum of the PC contents at the time of instruction execution and the (sign-extended) 16-bit displacement given in the next 16-bit word, namely $045c. At this point PC contains address of extension word, $ so JSR is to $ $045c = $7176b0 This should be the address of the first instruction of the subroutine, getchar. Note the use of parentheses: (d16,pc) indicates indirect addressing. This particular construct allows the code to be relocatable (doesn t reference specific memory locations directly). The MAS program has set all this stuff up for us using the xref directive (when it loaded the code into memory)

9 Second and Third Instructions In similar fashion, we can decode move.b do, msg In this case, the EA decodes to (d16, An) where An is address register A5. A5 contains $0070ee80 and d16 = $0004 so the contents of d0 goes to location $0070ee84 For Mac 68K programming, A5 is used as the base address for the data area: all data are stored in an area in memory starting at (A5). (Note use of parentheses for indirect address: read as the contents of A5. ) For things to work properly, you must not alter A5. And jsr stop is a subroutine call similar to the first

10 Lewis Carroll on Indirection From Through the Looking Glass, Ch. 8 (1871) "It's long," said the Knight, "but it's very, very beautiful. Everybody that hears me sing it -- either it brings the tears into their eyes, or else--" "Or else what?" said Alice, for the Knight had made a sudden pause. "Or else it doesn't, you know. The name of the song is called 'Haddock's Eyes.' " "Oh, that's the name of the song, is it?" Alice said, trying to feel interested. "No, you don't understand," the Knight said, looking a little vexed. "That's what the name is called. The name really is 'The Aged, Aged Man.' " "Then I ought to have said 'That's what the song is called'?" Alice corrected herself. "No, you oughtn't: that's quite another thing! The song is called 'Ways and Means': but that's only what it's called, you know!" "Well, what is the song, then?" said Alice, who was by this time completely bewildered. "I was coming to that," the Knight said. "The song really is 'A-sitting On A Gate': and the tune's my own invention."

11 Program with Loop and Array Input 20 characters into array using loop:

12 Program with Loop and Array II The assembly language program:

13 Commentary on Program Setup and avoiding the dreaded off-by-one problem

14 More on DBRA Further Commentary Fancy postincrement indirect move instruction

15 Now for Something Slightly More Ambitious Input 20 characters, sum up any integers, print resulting string and sum, as follows: Do the necessary initialization read the character Then for each of the 20 characters put the character in the msg array check if it represents a number in the range 0-9 if so, add it to a running total Now that we have all the input, print the array on the screen as a character string print the sum of the integers stop

16 Main Program With Subroutine Subroutine addit is not external follows on next page

17 The Subroutine and Data Area Subroutine addit follows last statement on previous page It is followed by the data area.

18 Branch Instructions and Comparisons Figures from M68000 Microprocessors User s Manual 9th Ed., 1993 Motorola Inc.

19 More Complex CCR Tests

20 Subroutines and Stack I

21 Subroutines and Stack II

22 Using the Stack for Other Purposes I

23 Using the Stack for Other Purposes II

24 Exercises