IBM PC Architecture - Registers and Basic Instruction Execution in DEBUG

By adding to the DEBUG commands previously learned, it is possible to examine and modify the contents of registers, assemble and unassemble mnemonic operation codes, and execute program instructions in either a single-step or continuous mode. We can create complete, executable programs using DEBUG.

DEBUG Register Exploration

Debug review
- run from the MS-DOS prompt as DEBUG or DEBUG filespec
- Q to Quit
- D or Daddress to display a block of memory starting at either the "current" display address or the address specified.

Examining Registers with Debug

Use R to display all registers and their contents. Example:

-R
AX=0A07  BX=FF00  CX=0000  DX=1818  SP=0080  BP=0000  SI=0000  DI=0000
DS=1398  ES=1398  SS=1983  CS=13A8  IP=000C   NV UP EI PL NZ NA PO NC
13A8:000C 8B2E0200      MOV     BP,[0002]                          DS:0002=A000
-

Modifying Register contents with Debug
- Use Rregister_id where "register_id" must be a 16-bit register name; current value is displayed; enter new value. Example:
```
-RAX
AX 0A07
:01F4
-
```

Using DEBUG with Assembly Language

Intel Assembler Operand Types

Note that although the following includes "labels" as valid "operand types", "labels" are not useable in DEBUG; these references are maintained here only for future use with true "assembler" programs (such as Arrow, MASM, or TASM).

reg: - any general or 8-bit register e.g. CX, DI, AL
reg8: - any 8 bit register e.g. BH, DL
reg16: - any (16-bit) general register e.g. AX, SI, BP
segreg: - any segment register e.g. CS, ES
mem: - a memory reference; possibly a label (not in DEBUG), an index or base register reference, a constant (in certain cases), a combination of the above (with optional segreg) e.g. TOTAL, [BX], TABL[BX], [ES:0080h], NUM+4 (again, note that labels are not available within DEBUG; you can only use register names and/or numbers).
regmem: - any general or 8-bit register or memory reference e.g. AH, CX, [DS:SI], COUNT
constant: - a constant expression e.g. 10h, 0, 'A', offset LABELX (not in DEBUG)
label (not in DEBUG): - the label associated with a statement or data definition e.g. TOTAL, STEP_2

Some simple instructions (MOV, ADD, etc)

In the following with respect to flags set, only the Carry, Overflow, Sign and Zero flags will be considered.

MOV [destination],[source]

copies the contents of the [source] into the [destination]
MOV segreg,regmem
MOV regmem,segreg
MOV regmem,reg
MOV regmem,constant
MOV reg,regmem

Flags: (none)

ADD [destination],[source]

adds contents of the [source] to contents of the [destination] and replaces the contents of the [destination] with the sum
ADD AL or AX,constant
ADD regmem,reg
ADD regmem,constant
ADD reg,regmem

Flags: Carry,Overflow,Sign,Zero

SUB [destination],[source]

subtracts contents of the [source] from the contents of the [destination] and replaces the contents of the [destination] with the difference
SUB AL or AX, constant
SUB regmem,reg
SUB regmem,constant
SUB reg,regmem

Flags: Carry,Overflow,Sign,Zero

Assembly and Unassembly in DEBUG

Use A or Aaddress to assemble lines of mnemonic code starting from the "current" address or the specified address until a blank line is entered; note that neither label references nor comments are allowed.
Use U or Uaddress to translate into mnemonic form the instruction values found in memory starting at the "current" address or at the specified address.

example:

-A0120
13A8:0120 mov ax,23D6
13A8:0123 mov bx,7BA9
13A8:0126 add ax,bx
13A8:0128
-u0120
13A8:0120 B8D623        MOV     AX,23D6
13A8:0123 BBA97B        MOV     BX,7BA9
13A8:0126 01D8          ADD     AX,BX
13A8:0128 000EFF16      ADD     [16FF],CL
13A8:012C 860F          XCHG    CL,[BX]
13A8:012E 33C0          XOR     AX,AX
13A8:0130 8BF0          MOV     SI,AX
13A8:0132 B92F00        MOV     CX,002F
13A8:0135 90            NOP
13A8:0136 FC            CLD
13A8:0137 0204          ADD     AL,[SI]
13A8:0139 80D400        ADC     AH,00
13A8:013C 46            INC     SI
13A8:013D E2F8          LOOP    0137
13A8:013F 2D370D        SUB     AX,0D37
-

Single Step Program Execution

Use T or P to "Trace" or "Proceed" to the next instruction. The difference between these two is in how subprogram calls are handled; "trace" follows the instruction sequence into any called routine; "proceed" does all of the instructions of the called routine before returning for the next DEBUG command; for most instructions "trace" and "proceed" perform identically. Example:

-Rip
IP 0FA9
:0120
-R
AX=0F29  BX=3A00  CX=0000  DX=0000  SP=FFE8  BP=0000  SI=0000  DI=0000
DS=1373  ES=1373  SS=1373  CS=1373  IP=0120   OV UP DI NG NZ NA PO NC
1373:0120 B8D623        MOV     AX,23D6
-T

AX=23D6  BX=3A00  CX=0000  DX=0000  SP=FFE8  BP=0000  SI=0000  DI=0000
DS=1373  ES=1373  SS=1373  CS=1373  IP=0123   OV UP DI NG NZ NA PO NC
1373:0123 BBA97B        MOV     BX,7BA9
-T

AX=23D6  BX=7BA9  CX=0000  DX=0000  SP=FFE8  BP=0000  SI=0000  DI=0000
DS=1373  ES=1373  SS=1373  CS=1373  IP=0126   OV UP DI NG NZ NA PO NC
1373:0126 01D8          ADD     AX,BX
-T

AX=9F7F  BX=7BA9  CX=0000  DX=0000  SP=FFE8  BP=0000  SI=0000  DI=0000
DS=1373  ES=1373  SS=1373  CS=1373  IP=0128   OV UP DI NG NZ NA PO NC
-