Debug: A--Notes

Using mnemonics

The segment-override mnemonics are cs:, ds:, es:, and ss:. The mnemonic for the far return is retf. String-manipulation mnemonics must explicitly state the string size. For example, use movsw to move word strings (16 bits), and use movsb to move byte strings (8 bits).

Assembling jumps and calls

The assembler automatically assembles a short, near, or far jump or call, depending on byte displacement, to the destination address. You can override such a jump or call by using a near or far prefix, as the following example shows:

-a0100:0500
0100:0500 jmp 502 ; a 2-byte short jump
0100:0502 jmp near 505 ; a 3-byte near jump
0100:0505 jmp far 50a ; a 5-byte far jump

You can abbreviate the near prefix to ne.

Distinguishing word and byte memory locations

When an operand can refer to either a word memory location or a byte memory location, you must specify the data type with the prefix word ptr or the prefix byte ptr. Acceptable abbreviations are wo and by, respectively. The following example shows the two formats:

dec wo [si]
neg byte ptr [128]

Specifying operands

Debug uses the common convention that an operand enclosed in brackets ([ ]) refers to a memory location. This is because Debug cannot otherwise differentiate between an immediate operand and an operand that is a memory location. The following example shows the two formats:

mov ax,21 ; load AX with 21h
mov ax,[21] ; load AX with the
; contents of
; memory location 21h

Using pseudoinstructions

Two popular pseudoinstructions are available with the a command: the db opcode, which assembles byte values directly into memory, and the dw opcode, which assembles word values directly into memory. Following are examples of both pseudoinstructions:

db 1,2,3,4,"THIS IS AN EXAMPLE"
db 'THIS IS A QUOTATION MARK: "'
db "THIS IS A QUOTATION MARK: '"
dw 1000,2000,3000,"BACH"

More Information About Debug: A

Debug: A--Examples

Debug: A