TUTORIAL: Assembler/Editor, pt. 2From: Michael Current (aa700@cleveland.Freenet.Edu)
Date: 01/18/92-12:43:29 PM Z
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From: aa700@cleveland.Freenet.Edu (Michael Current) Subject: TUTORIAL: Assembler/Editor, pt. 2 Date: Sat Jan 18 12:43:29 1992 Reprinted from A.C.E.C. BBS (614)-471-8559 DEBUGGING Your assembly language programs are bound to have bugs in them. Asm/Ed provides a method for testing assembled object code. When at the Asm/Ed EDIT prompt, type BUG and press return. The next prompt will be DEBUG. The commands for the debugger are all short one or two letter commands, some followed by an optional hexadecimal address. To exit DEBUG type X and press return. DR The DR command is used to display the contents of the 6502 registers: DR A=BB X=10 Y=20 P=B0 S=DF A is the accumulator, X and Y are the index registers, P is the processor status register (which includes the carry flag, zero flag, and etc.), and S is the stack pointer. CR The CR command is used to change the contents of any of the 6502 registers. CR<,1,2,,DE The values specified go into the registers in the same order they are displayed by the DR command. In the above example the accumulator is unchanged, the X register receives a 1, the Y register a 2, the status register remains unchanged, and the stack pointer is adjusted to DE. D The D command is used to display memory. D3000,0 tells the debugger to display memory location 3000 hexadecimal. When the second parameter is less than or equal to the first, only one location is shown. D3000,3010 requests the debugger to display memory from locations 3000 through 3010. Enter D by itself and the next 8 locations (3011 through 3018) will be displayed. If only the second parameter is omitted, a default of 8 memory locations are displayed: D3000 3000 10 40 20 22 34 11 12 FE Note that the output of the debugger is always in hexadecimal. All input addresses and register values are to be specified in hex as well. C The C command is used to change memory. The format is: C3034<21,23,,2E The command is immediately followed by the starting hexadecimal address to change. The values to be placed in memory, starting at the first location, are separated by commas. Two commas in a row tell the debugger to skip over that memory location, leaving it unchanged. In the above example memory location 3034 receives 21, 3035 receives 23, and 3037 gets 2E. You may use the D command to display memory just changed, to verify the new values. M The memory move command, M, is used to copy a block of memory from one area to another. The command format is: Mmmmm<yyyy,zzzz This tells the debugger to move memory from locations yyyy through zzzz to memory beginning at location mmmm. The destination address (mmmm) must be less than the first source address (yyyy) or greater than the ending source address (zzzz). If the source and destination areas of memory overlap, you may get unexpected results. V The verify memory command, V, is used to compare two blocks of memory. You might use this to compare two slightly different versions of the same program to see where something has changed, for example. The format is much like the move command: Vmmmm<yyyy,zzzz The above command tells the debugger to compare memory from yyyy through zzzz to memory at mmmm. Any memory locations that do not match are shown side by side, as follows: V7000<7100,7123 7101 00 7001 21 In the above comparison all memory from 7100 through 7123 matched memory from 7000 through 7023, except at one location. Memory location 7101 contained a 0 while 7001 contained a 21. L The list memory with disassembly, L, command is one of Asm/Ed's most powerful. It can be used to disassemble your operating system ROM (beginning at $C000), to see what some of the routines look like. It can be used to disassemble object files loaded into memory, to see how they work. The command format is: L7000 - List a screen (20 lines of code) of memory beginning at memory location 7000 L - List a screen of memory with disassembly starting at the next location (pick up where the previous L command left off) L7000,7000 - These three forms disassemble one instruction at memory location 7000 only L7000,0 L7000,6000 L2300,2400 - Disassemble memory and list to the screen from locations 2300 through 2400 When the debugger comes across data that will not disassemble into code (such as data tables or strings, for example) it will print a series of question marks to the display. Otherwise, the data is shown in hexadecimal, as well as in its equivalent assembly mnemonic form: L5000,00 A9 8A LDA #$8A A You may assemble one instruction at a time into memory with the debugger's A command. This comes in handy when you want to test a small patch to a program. Simply type A and press return to get into the single line assembly mode. You must first specify an address followed by a less than character (<), and then the assembly instruction. To assemble to successive memory locations, subsequent entries require only the less than character followed by the assembly instructions. For example: A [RETURN] 5001<LDY $1234 [RETURN] 5001 AC3412 Computer's response <INY 5004 CB Computer's response In the above example we have assembled LDY $1234 and INY into consecutive memory, starting at 5001 hex. Note that here your assembly instructions must use the dollar sign to indicate hexadecimal. Press return on an empty line to exit the mini assembler. You cannot refer to labels in the program, since the debugger doesn't keep track of them. If you do not know the absolute address of a label required for reference, then it is time to go back to the source code, make the changes, and reassemble. G The debugger can be used to go to any address and begin executing your code with the go command, G. Type the letter G followed by the first execution address. The program will continue to run until the system crashes, you press the break key, or a BRK (break) instruction is executed. T Sometimes you need to test a few instructions at a time. This is where the debugger's trace command, T, comes in. Type the letter T followed by the address at which to begin execution. The instruction will be executed, immediately followed by a dump of the instruction (list a single line with disassembly), and the cpu registers. This continues until a BRK instruction is executed, or you press the break key. S Sometimes you need to test a single instruction at a time. The debugger's step command, S, is used for this task. Enter S followed by the address to begin execution. The effects are the same as the trace command, except that the debugger stops execution after each assembly instruction. Type S and return repeatedly to continue "single stepping" the program. X The X command is used to exit the debugger and return control to Asm/Ed's editor. Error Codes The error codes between 128 and 255 are the same as those in your Atari BASIC reference manual. These are generally input, output errors associated with CIO (central input output) utility operations, the heart of your Atari's operating system. There are 19 error codes that you may encounter while assembling or debugging your programs: 1 - The memory available is insufficient for the program to be assembled. 2 - For the command "DEL xx,yy", the line number xx cannot be found. 3 - There is an error in specifying an address (mini- assembler). 4 - The file named cannot be loaded (wrong file format). 5 - Undefined label reference (you probably misspelled a label in your program). 6 - Error in syntax of a statement (missing operand, or misspelled assembly mnemonic). 7 - Label defined more than once. 8 - Buffer overflow. (I'm not certain what this means.) 9 - There is no label or * before "=". (An equal sign was found in the first field of a line of code. All equals must be preceded by either a valid label or the asterisk.) 10 - The value of an expression is greater than 255 where only one byte was required. (e.g. LDA #LABEL, where label is an address of some memory location greater than 255.) 11 - A null string has been used where invalid. 12 - The address or address type specified is incorrect. (e.g. LDA (PGZRO),Y would result in this assembly error if the label PGZRO was not an address of a memory location less than 256.) 13 - Phase error. An inconsistent result has been found from pass 1 to pass 2. (e.g. Two bytes were reserved for some label on the first pass, but on the second pass only one byte was needed. This is avoided by minimizing forward references, and defining all known labels at the top of the file before any assembly code. You will get this error a lot, as you learn the language.) 14 - Undefined forward reference. (e.g. Misspelled label, or reference to a label not defined.) 15 - Line is too large. 16 - Assembler does not recognize the source statement. 17 - The line number is too large (32767 is maximum). 18 - LOMEM command was attempted after other command(s) or instruction(s). LOMEM, if used, must be the first command after entering the Asm/Ed editor. 19 - There is no starting address. (e.g. You forgot the *= directive at the top of your program.) Expressions The assembler can perform many useful computations for you. The operators recognized and operations they perform are as follows: + Addition - Subtraction * Multiplication / Division & Logical and Expressions may not contain parenthesis, and they are always evaluated left to right. (There is no precedence placed on operators). Some examples follow: 100 STORAGE = $4000 110 *= STORAGE + $10 ; Set program counter to $4010 ... 200 JMP START+20 ... 300 LDA #STORAGE&$0FF ; Get low byte of STORAGE in A 310 LDX #STORAGE/$100 ; Get high byte of STORAGE in X ... 320 LDA #3*15 ; Load the number 45 into A USR Routines The USR command of Atari BASIC allows you to call assembly language routines. These routines can perform special functions to vastly improve the performance of BASIC. For example, assembly USR routines may be implemented for player missile graphics movement, sort algorithms, or high speed disk I/O functions. Assembly code won't normally be loaded as part of your BASIC program. It must be loaded using a routine in BASIC by placing the data values into strings, or POKEing it into safe RAM, for example. You may place up to 256 bytes of assembly code into page 6 (beginning at memory location 1536). If you do not use the cassette (C:) then up to 128 bytes of code can go into page 4 (beginning at memory location 1024), the cassette buffer. If your code is "position independent" it may be loaded into a BASIC string. What is "position independent" assembly code? Such a program may have no JMP or JSR instructions (with the exception of JSR's to ROM addresses that are guaranteed not to move). So how to you implement loops? Use branch instructions. If your code gets much larger than 256 bytes, writing position independent code can be very difficult. The largest routine I've ever written of this type was 410 bytes long. You may also "relocate" your code. This requires a foreknowledge of all the JMP and JSR instructions in your code. You may then load the object code into a string, determine its starting address, and then POKE adjusted address values in for all the JMP and JSR instructions. This is no small task, and is seldom used. Generally, your USR routines will be fairly small and can be written in a position independent manner. The format of a BASIC USR command is: 10 A = USR(1536, PARAM1, PARAM2, PARAM3) The first parameter, 1536 above, must be the starting address of the assembly code you wish to execute. The values following are parameters which are passed to the assembly code on the system stack, after being converted to integer. The variable A takes on an integer from memory locations $D4 and $D5 (low byte, high byte). This is the mechanism you would use to return a value to BASIC. Lets write a USR routine to add two integers, and return the result. Our BASIC program might look like this: 10 TRAP 1000 20 OPEN #1,4,0,"D:MYUSR.OBJ":REM Our USR code in a file 30 TRAP 70 40 FOR I=1 TO 6:GET #1,A:NEXT I:REM Ignore 6 byte load header of file 50 I=1536:REM USR routine was assembled for page 6 60 GET #1,A:POKE I,A:I=I+1:GOTO 60:REM End of file error will terminate our entry of the program 70 CLOSE #1 80 PRINT "INPUT NUMBER 1 ";:INPUT N1 90 IF N1<0 OR N1>65535 THEN ? "OUT OF RANGE":GOTO 80 100 PRINT "INPUT NUMBER 2 ";:INPUT N2 110 IF N2<0 OR N2>65535-N1 THEN ? "OUT OF RANGE":GOTO 100 120 SUM = USR( 1536, N1, N2 ) 130 PRINT "NUMBER ";N1;" PLUS ";N2;" EQUALS ";SUM 140 END 1000 PRINT "COULD NOT FIND USR ROUTINE FILE" 1010 PRINT "MYUSR.OBJ" 1020 END Now we need to write an assembly language program with Asm/Ed that implements this USR routine. It will accept parameters N1, and N2 off the stack (two, two byte integers), add them, and return the result to SUM through memory locations $D4 and $D5. Our code might appear as follows: 0 ;LIST#D:MYUSR.ASM 10 ;ASM ,,#D:MYUSR.OBJ 11 SUM = $D4 12 NUM1 = $E0 13 NUM2 = $E2 20 *=1536 ; Assemble for PAGE 6 30 ADDTHEM PLA ; First off the stack is parameter count 40 BEQ ERROR ; Always check for no parameters ERROR 50 CMP #2 ; Did we get exactly 2 parameters? 60 BEQ AOK 70 TAX ; No, clean up stack and return safely 80 CLEANUP PLA ; Two bytes per parameter 90 PLA 100 DEX ; Get all the parameters off? 110 BNE CLEANUP ; when all gone, just the valid return addr 120 ERROR RTS ; is at the top of the stack for the RTS 130 ; We have valid input, compute the sum 140 ; The first parameter in the USR call (after the addr) 150 ; is the first parameter off the stack, high byte 160 ; low byte sequence. REMEMBER this! 170 AOK PLA ; Get NUM1, high byte 180 STA NUM1+1 190 PLA ; Get NUM1, low byte 200 STA NUM1 210 PLA ; Get NUM2, high byte 220 STA NUM2+1 230 PLA ; Get NUM2, low byte 240 STA NUM2 250 ; Now we have the data in temporary storage 260 ; and the stack is cleared of parameters. 270 ; Just the return address (to get us back to BASIC) 280 ; is at the top of the stack - which gets pulled off 290 ; into the program counter automatically by the RTS 300 ; instruction. 310 CLC ; Must clear the carry flag first 320 LDA NUM1 ; Low byte of first integer to add 330 ADC NUM2 ; Add to low byte of second integer 340 STA SUM ; And store in low byte of their SUM 350 LDA NUM1+1 ; Now add high bytes, leave carry alone 360 ADC NUM2+1 ; It "carries over" from previous add 370 STA SUM+1 ; And their summation is complete 380 RTS ; Back to BASIC Enter this program with Asm/Ed and execute the instructions in the first two comment lines. When you get an assembly with no errors, your file D:MYUSR.OBJ should be ready to test with the first BASIC program. Work at this until it performs as expected. As you become more adept at writing USR routines, you may wish to develop utilities for converting OBJ files into a series of BASIC DATA statements, so you can simply READ and POKE them without using messy file I/O to initialize the USR routine. It takes a relatively long time to install USR routines by poking them into memory or strings, but once in place they execute amazingly fast. You will find that USR routines are incredibly difficult to debug. You need to initialize them and call them from BASIC. If you mess up the stack or some other operation, the computer usually crashes inexplicably. It isn't easy to debug USR routines from DEBUG, because you will have to write sophisticated test routines to stuff all sorts of test values on the stack. Stand Alone Assembly Sooner or later you will get tired of USR routines (mostly because they are so difficult to debug). When you do, it is time to take the plunge into writing a stand alone assembly language program. Then you will get into the complexities of keyboard input, screen output, disk I/O, and printer output from the Asm/Ed environment. Complete libraries of routines, such as a "graphics package" that performs the equivalent of BASIC's GRAPHICS, COLOR, PLOT, and DRAWTO, will become a necessity. This is where BOOT CAMP will help the most. In the months to come you will learn everything from keyboard input to floating point processing, all from the assembly language level. Most of our listings are in Mac/65 format. With the exception of macros (Asm/Ed is not a macro assembler), most changes to Asm/Ed compatibility will be minor. As an example of a stand alone assembly language program, and an illustration of its raw speed, we present the following demonstration. First type this BASIC program and run it. While it executes (it will take about 12 minutes), read the remainder of this article to see how the same functions can be performed in assembly language: 10 DINDEX=88:REM Screen RAM pointer 20 SCREEN=PEEK(DINDEX)+256*PEEK(DINDEX+1) 30 FOR X=0 TO 255 40 A=X 50 FOR Y=0 TO 255 60 POKE SCREEN+Y,A 70 NEXT Y 80 NEXT X At location DINDEX is a two byte "pointer". Memory locations 88 and 89 hold the address of the beginning of screen RAM. The equation in line 20 calculates the variable SCREEN, which we use as a direct pointer, for the POKE in line 60. In our assembly language equivalent of the above program, this problem is even EASIER to solve. (This is seldom the case however, most things are much harder to do in assembly language. This demonstration is designed purposefully to show the strengths and speed of assembly language.) Next two loops are setup. The inner Y loop is used to poke the current value of X into the first 256 screen RAM locations. You will see these characters fill the top portion of your display. All ATASCII values from 0 through 255 are poked, with the help of the X loop. The variable A was used simply for a more symmetrical comparison with the assembly code to follow. Let this BASIC program run to completion. Time it carefully, study the sweep second hand of your watch creep slowly along. Feel the annoying impatience of this terribly slow program creep up your spine. When you finally get the READY prompt, reboot your computer with Asm/Ed and enter this equivalent assembly language program: 0 ;LIST#D:SCREEN.ASM 1 ;ASM,,#D:SCREEN.OBJ 2 *=$3400 3 RUNAD=$2E0 10 DINDEX = 88 ; Screen RAM pointer 20 ; We don't have to compute SCREEN, we use post indexed addressing 30 START LDX #0 ; Initialize variables for loops 40 LDY #0 50 STORE TXA ; Place screen character into A register 60 PUTIT STA (DINDEX),Y ; Place character on screen 70 INY ; Next screen location 80 BNE PUTIT ; Y register "wraps around" to zero after 255 90 INX 100 BNE STORE ; NEXT X 110 RTS ; Return control to DOS 120 *= RUNAD 130 .WORD START ; So we can load and run from DOS Now execute the two commands in the first two comment lines at the top of the listing. If you get no assembly errors then you will have a file SCREEN.OBJ that is ready to load and run. Go to DOS and execute a binary 'L'oad of the file SCREEN.OBJ. It will run immediately after loading and return control back to DOS after performing all 65,536 "POKES" of characters to screen memory. Did you catch it? You probably didn't if you blinked. This version of the program takes barely a second to run! If you want to watch the show for a while, and exit to DOS when a KEY is pressed, for example, modify your program as follows: 15 CH = 764 ; Keyboard buffer ... 101 LDA #255 102 CMP CH ; key pressed? 103 BEQ START ; Nope, loop 104 STA CH ; Yes, clear out key buffer and exit to DOS List this version to disk and reassemble it. When loaded from DOS, it will "poke" all those ATASCII patterns to the screen continuously until you press any key on the keyboard. To RANDOMIZE the show, make these changes: 16 RANDOM = 53770 ; Always a random number here ... 50 STORE 60 PUTIT LDA RANDOM ; Get a random fill character 61 STA (DINDEX),Y ; Place character on screen Notice how I always added a meaningful label for each important memory location. Avoid the use of code such as LDA 53770. The proper use of labels makes it much easier to see exactly WHAT your program does and HOW it gets the job done. If you didn't pay much attention to ANALOG's Master Memory Map series, I strongly recommend that you go back and read it all. Even if you do not understand all of it, you will learn a lot. A good memory map is the key to unleashing all the power of your computer. Compute!'s Mapping The Atari, Revised Edition is also a very good reference guide. As a 6502 Assembly language reference manual, I use 6502 Assembly Language Programming by Leventhal. This is a general reference for the 6502 microprocessor, and does not have any specifics on the Atari computer. It does detail all the 6502 assembly mnemonics, and provides examples of multiply, divide, and other useful routines. When you find that Asm/Ed is too slow to suit your tastes, as you build larger and more sophisticated programs, consider upgrading to Mac/65. This macro assembler supports the use of INCLUDE files, allowing you to easily import "canned routines" that have already been debugged. It's MACRO capabilities allow you to define high level constructs that vastly simplify the development of assembly programs. With a good MACRO library (such as OSS/ICD's Mac/65 Toolkit or QuickCode from Stardust Software), your assembly source code will resemble BASIC or some other high level language, while retaining all the power and speed of pure assembly language. Mac/65 is the absolute FASTEST native 6502 assembler I have ever used, bar none. (Mad Mac for the Atari ST will assemble 6502 code that blows the doors off Mac/65; but that's a whole new ball game.) Welcome to the fast and complicated world of assembly language programming. I hope this guide will inspire you to put that inexpensive Asm/Ed cartridge to work on all those fantastic ideas that old faithful Atari BASIC never could handle. By: Matthew J. W. Ratcliff, Ratware Softworks, 32 S. Hartnett Ave., St. Louis, MO 63135 -- Michael Current, Cleveland Free-Net 8-bit Atari SIGOp -->> go atari8 <<-- The Cleveland Free-Net Atari SIG is the Central Atari Information Network Internet: firstname.lastname@example.org / UUCP: ...!umn-cs!ccnfld!currentm BITNET: currentm%carleton.edu@interbit / Cleveland Free-Net: aa700
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