CHAPTER 2 D86 DEMONSTRATION To demonstrate some of the powers of D86, let's walk through a D86 session. 1. Make sure your current directory contains all the files A86.COM, D86.COM, and HEXOUT.8. 2. Assemble the HEXOUT program by typing the command A86 HEXOUT.8. The A86 assembler will create the files HEXOUT.COM and HEXOUT.SYM. Look over the listing of the program, to get acquainted with what it does. 3. Type the command D86 HEXOUT 41 42 5A followed by the ENTER key. Everything following the "D86 " in the line you just typed is what you would have typed if you had invoked HEXOUT without the debugger-- 41 42 5A are the hex codes that HEXOUT will turn into ABZ and send to the console. When the debugger takes control, the screen should blank; and the D86 debugger screen should appear. The blinking cursor should be at the bottom left. A sign-on message should appear at the upper right. A disassembly of the HEXOUT program should be in the upper left. The label HEXOUT should appear on the first line, followed on the second line by the address 0100 and the instruction MOV SI,TAIL_BUFF. To the left of the address should be a reverse video hash sign. If you have a CGA monitor, the hash sign will blink, to compensate for the fact that the reverse video isn't as obvious in the lower resolution of CGA. 4. Notice the display of register values in the lower left corner. The values are all 4-digit hexadecimal. At the top of the second column of registers is a sequence of lower case letters. This is the flags display. Each small letter stands for a flag whose value is currently TRUE. The flags settings are those that were handed to D86 by the operating system starting the program; for MSDOS V3.1, the settings are "i z e". That display indicates that the interrupt flag "i", the zero flag "z", and the parity-even flag "e" are all TRUE; the other flags are FALSE. To the right of the registers are six lines labelled 1: through 6:. These are the memory window lines. Since you haven't specified any memory windows yet, they contain nothing but their numbers. Below the memory window lines is a line labelled 0:. This is the stack display line. The number 0: gives the number of words on the stack, currently zero because nothing has been pushed onto the stack. 5. Observe that the sign-on message tells you to press Alt-F10 for help. Do so (that is, hold down the Alt key while pressing the F10 key). You are now in help mode, where you will remain until you press Alt-F10 again. D86 will keep changing the help window, depending on what it thinks you are doing. Right now you have a summary of the main function keys, plus a few other things. Press F10 (without the Alt), and you'll get a summary of one-letter debugger commands. Press F10 a second time and you'll get a summary of Ctrl-key commands. Finally, press F10 a third time to return to the function-key help screen. 2-2 6. Let's try an immediate assembly language instruction. Press the "M" key, which is the first letter of the immediate instruction MOV AX,123. Note that the reverse video block jumps from the hash sign within the disassembly, down to the line just above the blinking cursor. The block is the debugger's cursor; the blinking cursor is the program's console output cursor. The debugger does not use the blinking cursor because we do not want the program's output to interfere with the debugger's output. Also note that the help window is now telling you that you are typing in an assembly language line. 7. Complete the line MOV AX,123 followed by the ENTER key (from now on I'll assume that you know that lines are followed by the ENTER key, and that any periods at the end of a line are part of my sentence, and not part of what you type). The debugger immediately executes the assembly language line you just typed, setting the register AX. Note that you did not have to learn a debugger command for setting registers; if you know A86, you already know how to set registers! The value of AX is now 007B, which may surprise you if you expected 0123. A86's default base is 10, so 123 was taken as decimal; which is hex 7B. Type MOV AX,0123 instead, to get a value of hex 123. 8. Let's now play with the flags display. Type the line ADD AL,05D, which changes AL (the last two digits of AX) to hex 80, and alters the flags to "o is a ". The interrupt flag is still on; but zero and parity-even are now off. They have been replaced by "o" overflow, "s" sign, and "a" auxiliary carry. 9. Type the line consisting of just CMC. This is the Complement Carry instruction. Observe that the "c" appears. Notice also that the CMC that you typed remains on the screen. Notice on the help window the entry "F3 RepeatCmd". This tells you that the F3 key will repeat the last line command (not function key) that you typed. Press F3 several times, to see the carry flag toggle on and off. Isn't that the cleanest flags display you've ever seen? 10. Let's single step an instruction. Press the F1 key. This executes the program instruction, loading the SI register with TAIL_BUFF. The disassembly cursor moves down to the next instruction. Observe that SI has changed to 0081, which is the pointer to the invocation command tail, which should contain the string typed after HEXOUT: " 41 42 5A" followed by a carriage return code (hex 0D). 2-3 11. Let's examine memory to verify that last assertion. Press the "1" key. The cursor jumps to the start of memory window 1, and the help window gives you a huge choice of memory types to display. The entry "ByteHex 2" tells us that "B" will cause hex bytes to be displayed. The "2" indicates that the display occupies a fixed number of display bytes for every memory unit, namely 2 hex digits. Type B followed by a comma, to indicate that you want nothing but hex bytes to be displayed. Now the help window asks for a segment location. Let's use the DS register: type DS followed by a comma. Now the help window wants an offset within the segment: type SI. Before typing the terminating ENTER, backspace out what you have typed, and watch the help windows regress appropriately. Isn't that impressive? Now retype the line, "B,DS,SI". Note that when you press ENTER, the line fills out with hex values: 20 34 31 20 34 32 20 35 41 0D etc. (61 instead of 41 is OK; it means you typed the invocation in lower case.) 12. Let's look at the same line, displayed as text. Type "2", moving to memory line 2, then type the line "T,SI". This time you specified type T for text, and you left out the segment register specification. D86 uses DS when you leave out the segment register; so in this case you'll get the same segment. This time the display starts with "41 42 5AM"; the "M" is the carriage return, which is control-M, ambiguously displayed. You can read Chapter 6 later on for descriptions of all the types, including other text types allowing non-ambiguous displays. 13. Let's execute the next instruction, CALL GET_HEX. Here we have a choice, between executing the procedure all at once, or stepping into the procedure to execute its instructions one at a time. Let's try stepping in first: type the F1 key. The cursor jumps to location GET_HEX, on the same disassembly screen. The SP register decrements from FFFE to FFFC, and a value 0106 appears on the stack. This is the return address, pointing to the instruction following CALL GET_HEX. 14. Watch memory lines 1 and 2 as you press F1 again, single stepping the LODSB instruction. You had set up the lines to be pointed to by SI. Since SI changes when LODSB is executed, the memory displays advance to the next byte. Note that the AL register contains the value hex 20, a blank. 15. In a normal debugging session, we would continue stepping within GET_HEX, but let's not do that right now. Instead, press the F6 "TrapRet" key, which starts the program going, trapping at the return address on top of the stack, which was 0106. The cursor jumps back up to location 0106, the value is no longer on the stack, and SI and the memory displays have advanced to 0084. 2-4 16. Let's try the classic "G" command, common to all debuggers. Type the line "G,0103", noticing the help windows as you go along. After you press ENTER, the program runs until it gets back to the trap address you provided, 0103. Note that the program has called OUT_VALUE to output the "A" that corresponds to your input hex 41. The "A" appears on the bottom line, and the blinking cursor advances. 17. Let's execute the next CALL GET_HEX all at once, by pressing the F2 ProcStep key. SI advances again, and AL is loaded with the next value 42. 18. Notice that the disassembly is symbolic: the display is CALL GET_HEX, not CALL 0112 as lesser debuggers might give you. Let's try symbolic input: type the line "B,HEX_DIGIT?", causing the debugger to set a fixed trap at location HEX_DIGIT?. Now set your program running with a simple G followed by the ENTER key. The program traps at HEX_DIGIT?. Since this location is not in the disassembly window, the window is regenerated, and the cursor placed at HEX_DIGIT?. The memory displays now point to the final number "5A". 19. Press F3 to repeat the G-command. The program traps at HEX_DIGIT? again, with SI advanced to the "A". Press F3 again; advancing SI to the final carriage return. 20. Press F3 yet again. Since HEX_DIGIT? is never reached again, the program runs to its completion. D86 automatically traps at the EXIT instruction: in this case, it is INT 021 with the AH register set to hex 4C, the function number for EXIT. If we try to start the program again from here, we will be frozen here: we must issue the Q command to exit the session. Don't do it yet, though. 21. Before exiting, let's check out HEX_DIGIT? more thoroughly. First, we clear the breakpoint we set, by typing "B" followed by the ENTER key. 22. Type the command line "J 0200", jumping to a scratch-pad memory area. Then press the F7 key, entering Patch Memory mode. The cursor moves into the disassembled instruction, signalling that whatever you type is clobbering it. 23. Type in the lines "INC BL", "MOV AL,BL", and "JMP HEX_DIGIT?". Press ENTER at the beginning of the fourth line, exiting Patch Memory mode. The cursor will return to the left of the 0200 INC BL line. 24. Type the immediate command "MOV BL,'0'-2". The BL register should change to the evaluated value, hex 2E. 2-5 25. Execute the patch subroutine by typing the line "CALL 0200". The value BL increments to 2F, which is one less than the lowest digit, '0'. The Carry flag is set, indicating that HEX_DIGIT? has correctly judged 2F not to be a hex digit. Now press F3 repeatedly, executing your patched subroutine for each decimal digit. The "c" will disappear as the values advance; and AL will hold the correct binary value for each hex digit BL. When BL reaches 3A, the "c" comes back on again, indicating that we are beyond the decimal digits. When BL reaches 41, "c" goes off, and AL values of 10 through 15 are displayed. When BL reaches "47" "c" comes on yet again, because G is not a decimal digit. Type "MOV BL,05F", followed by "CALL 0200", followed by F3 several more times to verify correct action for the range of lower case 'a' through 'f'. You have, relatively quickly, done a thorough test of HEX_DIGIT?. How long would that have taken on a lesser debugger? 26. Type Q followed by ENTER to exit the debugger.