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Text File | 1994-06-19 | 12.0 KB | 314 lines

The 6809 assembler/simulator/debugger ------------------------------------- Written by: L.C. Benschop Eindhoven The Netherlands. Introduction ------------ 6809sim contains a Forth assembler for the Motorola 6809 processor and an instruction-level simulator with full-screen memory editing and interactive debugging. There is also a disassembler. 6809sim started as a hobby project under F-PC in 1989. I was fascinated by the elegant and powerful instruction set of the 6809, but I did not have one. So I decided to write one myself. Forth was _the_ language of choice by then, so I write it in Forth. I started with the assembler. I wanted to have an assembler with normal syntax instead of a typical Forth assembler. In fact I wanted an assembler like that of F-PC. I succeeded in writing one. Further I wanted an interactive processor simulator with memory editing, a register display etc. I had seen something similar with a 6502. I got to have a 6809 version. In 1993 I ported the thing to PFE. At the same time I wrote a C-version of the simulator with no fancy interactive debugging, but it ran 40 times faster. I kept the simulators compatible, so they could run each other's programs. That helped me tremendously, both in finding bugs in any of the simulators and in debugging 6809 code. Most 6809 programs that I write or adapt today, are not written in Forth assembler but with a conventional assembler. One such program is a primitive port of E-Forth. It runs on the simulator, but it is assembled on a normal assembler. I know I *should* have a metacompiled Forth for the thing but I never finished writing one. 6809sim.4th is mostly ANSI compatible. It runs on pfe-0.95 and hopefully on later versions too. PFE can be obtained via ftp from roxi.rz.fht-mannheim.de pub/unix/languages/forth To load 6809sim, start pfe and type include 6809sim.4th After some messages of redefined words, the simulator is ready for use You start it by entering simulate You quit it by typing Q. The ? key gives you some help. The assembler _____________ The 6809 assembler is a Forth assembler whose syntax is as close to normal assembler as one can get. As the instructions are parsed by the normal Forth parser, there are some differences. The most important ones are: - There must be spaces between all components of an instruction. Therefore on writes LDA # $3E instead of LDA #$3E PSHS A, X, Y instead of PSHS A,X,Y STA 15 ,X instead of STA 15,X - The constant 0 is required in register indirect addressing. STB 0 ,S instead of STB ,S - No # in the instructions ANDCC ORCC and CWAI ANDCC $FE instead of ANDCC #$FE - Indirect mode is signalled by the word [] after the address instead of the address inside brackets. For example ADDA 1 ,U [] instead of ADDA [1,U] - Expressions are postfix. ADCB DATA1 14 + instead of ADCB DATA1+14 - Labels must be preceded by the word LABEL and only backward references to labels are allowed. LABEL SUBR ADDA 4 ,X STA 5 ,X RTS LABEL MAIN JSR SUBR instead of SUBR ADDA 4,X STA 5,X RTS MAIN JSR SUBR This way one writes the program bottom-up with the main routine last. If one must have a forward reference (e.g. to jump to the main routine) one can write a JMP -1 at the start and later patch the address using the word V! Like this. LABEL START JMP -1 ..... LABEL MAIN MAIN START 1 + V! \ Patch the address of main in the jump. - The Forth control structures BEGIN .. UNTIL BEGIN .. WHILE .. REPEAT IF .. THEN and IF .. ELSE .. THEN are allowed, like in Forth assemblers. These are translated to conditional and unconditional branch instructions. Allowable conditions are: 0= 0<> 0>= 0< < >= > <= U< U>= U> U<= VC (overflow clear) and VS (overflow set). Note that the conditions generate a branch whose mnemonic is the opposite of the condition, e.g. VC IF generates a BVS instruction. The following Forth assembler code 0>= if leax 1 ,x else leax -1 ,x then replaces the more conventional bmi lab1 leax 1,x bra lab2 lab1 leax -1,x lab2 - The words , and C, enter constant bytes and words, ALLOT reserves space $1244 , instead of FDB $1244 $55 C, instead of FCB $55 25 ALLOT instead of RMB 25 $100 ORG instead of ORG $100 An assembler source file must start with the word ASSEMBLE and end with the word ENDASM It is assembled from the Forth commandline with include filename or it can be assembled from the interactive simulator. The assembled binary result resides inside the 64 memory space of the virtual 6809. Two example assembler files are included, mirror.asm and bin2dec.asm. Loading, saving and disassembling of binary files. __________________________________________________ Once a source file is assembled, the binary result can be saved to disk. The word VSAVE expects an address and a length on the stack. The command $1000 $100 vsave foo saves 256 bytes starting at address $1000 into the file foo. We can assemble the file mirror.asm and save the binary code by. include mirror.asm 0 $19 vsave mirror.bin We could also add the VSAVE command to the assembler source file. The word VLOAD loads a binary file into the memory of the virtual 6809. It expects an address on the stack. The command 0 vload mirror.bin loads back the binary result of mirror.asm, without assembling first, The word DISAS disassembles a section of memory. It expects two addresses on the stack. Suppose the file mirror.asm was assembled. The command 0 $18 disas shows the following listing. LDY # $0040 \ 0000 10 8E 00 40 LDB ,Y+ \ 0004 E6 A0 STB $80 \ 0006 D7 80 LDX # $0081 \ 0008 8E 00 81 ABX \ 000B 3A TSTB \ 000C 5D BEQ $0016 \ 000D 27 07 LDA ,Y+ \ 000F A6 A0 STA ,-X \ 0011 A7 82 DECB \ 0013 5A BNE $000F \ 0014 26 F9 JMP $FFFF \ 0016 7E FF FF The interactive simulator _________________________ The Forth command SIMULATE starts the interactive simulator. This should show a HEX/ASCII display of the memory of the 6809 with a display of the processor registers at the bottom. Q leaves the simulator. ? shows a help screen. One can use the cursor keys and home/end/pgup/pgdn keys to move the cursor throughout the 64K memory space, of which 256 bytes are visible at a time. If these keys fail, one can use alternatively the Wordstar keys control-S control-D control-E and control-X as cursor keys, control-R and control-C for page up and down and control-A and control-F for begin and end of line. The space key moves also to the next location. The keys 0-9 and the letters A-F change the HEX value at the cursor location. The ASCII display changes accordingly, but it cannot be edited directly. The following one-letter commands manipulate the assembler file: N (asks for a name) sets the name of the assembler file. Z invoke the editor on the assembler file. Y assembles the assembler file. As an example, select the file mirror.asm (N command) and assemble it (Y command). Then move the cursor to the address $0040 and enter a string preceded by a count byte, like in the display below. 0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF 0000 10 8E 00 40 E6 A0 D7 80 8E 00 81 3A 5D 27 07 A6 ...@f W ...:]'.& 0010 A0 A7 82 5A 26 F9 7E FF FF 00 00 00 00 00 00 00 '.Z&y~ ....... 0020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ 0030 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ 0040 05 46 4F 52 54 48 00 00 00 00 00 00 00 00 00 00 .FORTH.......... 0050 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ 0060 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ 0070 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ 0080 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ 0090 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ 00A0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ 00B0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ 00C0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ 00D0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ 00E0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ 00F0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ CC=00000000 A=$00 B=$00 DP=$00 X=$0000 Y=$0000 U=$0000 S=$0000 EFHINZVC PC=$0000 LDY # $0040 The following commands execute instructions. T executes the instruction at the program counter location (that is displayed at the bottom). G asks for a breakpoint address and executes instructions until the breakpoint is encountered. S sets a breakpoint just after the instruction at the program counter location and starts running until that breakpoint. This way one can execute a whole subroutine instead of just a JSR instruction. 1HH101H Try to single-step the mirror program using the T command and watch the registers and the memory. The following commands are also useful in debugging. R asks for a register, type A,B,X,Y,U,S,P (for PC) C (for CC) or D (for DP) and asks for a value. Change a particular register. P sets the cursor location to the program counter location. X sets the program counter location to the cursor location. U sets the program counter to the next sequential instruction, without executing. A session of an X command followed by several U commands shows the disassembled instructions of a program fragment in the bottom of the display. Remove the string at address $80 by typing zeros over it. Now reset the program counter to location 0 (either by moving the cursor to location 0 and typing X, or using the R P command) and run the program using the G command. Give FFFF for breakpoint. Try to run the program with different strings at address $40. Move the cursor to address 0 and type X. Then type several U's in a row. Watch the disassembled instructions. The following commands are for file loading and saving. L (asks for address and name) loads a binary file like VLOAD W (asks for address, length and name) saves a binary file like VSAVE. The simulator executes all normal 6809 instructions, but the following instructions have special meanings. SWI2 prints the character in the B register to the screen. SWI3 reads a key from the keyboard (if available) into the B register. Resets carry flag if key available, sets carry if no key available. SYNC Exits the simulator with a beep. One can always interrupt the simulator with control-U. One can restart the simulator by retyping SIMULATE. Note that a program run interrupted by ^U cannot be properly continued, as an instruction may be interrupted halfway. Normally one would leave the simulator with Q. The program bin2dec.asm is slightly more complicated than mirror.asm It is assembled at address $100 so you must move one page down and you must set the program counter to $100. It converts numbers to decimal and prints them using SWI2. Run the program, single step it, step over subroutine calls using the S command. Look how the BCD number is generated at address $11B, watch the decrementing counters at addresses $120 and $121, see the stack pointer decrement and increment with subroutine calls and returns, and see the actual stack at $1FF. Related programs ________________ A C version of the simulator, a conventional 6809 assembler and some example programs (a tiny BASIC and E-Forth) were posted to alt.sources in May 94. These are archived at some sites, for example wuarchive.wustl.edu usenet/alt.sources/articles the files 10405.Z 10406.Z and 10407.Z They should be obtainable from nic.funet.fi too, but this site has a different archive organization. The example programs run also under the interactive simulator in Forth.