Bits: logic, shifts and rotates

Intermediate

The logic instructions work on each bit separately.

Instruction	A bit of the result is 1 when...	Common use
`AND d, s`	both bits are 1	clear bits: keep only some (a mask)
`OR d, s`	either bit is 1	set bits
`XOR d, s`	the bits differ	flip bits; `XOR AX, AX` clears AX
`NOT d`	the bit was 0	flip every bit
`TEST d, s`	(like AND, but only the flags change)	is a bit set?

AND, OR, XOR and TEST always clear CF and OF, and set ZF and SF from the result. NOT changes no flags.

Upper and lower case

In ASCII, a lower-case letter is its capital plus 20h: 'a' is 61h and 'A' is 41h. Only bit 5 (value 20h) differs, so:

To	Do	Mask
upper case	clear bit 5	`AND AL, 11011111B` (0DFh)
lower case	set bit 5	`OR AL, 00100000B` (20h)
the other case	flip bit 5	`XOR AL, 20H`

; Read a letter and print it in the other case.
.MODEL SMALL
.STACK 100H
.CODE
MAIN PROC
    MOV AH, 1
    INT 21H            ; AL = the letter typed
    MOV DL, AL
    XOR DL, 20H        ; flip bit 5: a <-> A
    MOV AH, 2
    INT 21H
    TEST DL, 20H       ; is bit 5 set now? (ZF = 0 means lower case)
    MOV AH, 4CH
    INT 21H
MAIN ENDP
END MAIN

Input used: q

Open in compiler

TEST AL, 1 checks the lowest bit: ZF = 1 means the number is even.

Shifts and rotates

Instruction	Moves the bits	The bit pushed out goes to
`SHL d, n`	left, 0s come in: × 2ⁿ	CF
`SHR d, n`	right, 0s come in: ÷ 2ⁿ (unsigned)	CF
`SAR d, n`	right, copies of the sign bit come in: ÷ 2ⁿ (signed)	CF
`ROL d, n` / `ROR d, n`	round in a circle	CF (and back in at the other end)
`RCL d, n` / `RCR d, n`	round in a circle through CF	CF

On the 8086 the count n is 1 or the register CL: MOV CL, 3 then SHL AX, CL. (StepVioz also accepts a number, as later processors do.)

Shifting left one bit at a time pushes each bit into CF, top bit first: a way to print a number in binary.

; Print a byte in binary: shift each bit into CF, top bit first.
.MODEL SMALL
.STACK 100H
.CODE
MAIN PROC
    MOV BL, 45         ; 45 = 00101101b
    MOV CX, 8          ; eight bits
NEXTBIT:
    MOV DL, '0'
    SHL BL, 1          ; the top bit goes into CF
    ADC DL, 0          ; '0' + CF: '0' or '1'
    MOV AH, 2
    INT 21H
    LOOP NEXTBIT
    MOV AH, 4CH
    INT 21H
MAIN ENDP
END MAIN

Open in compiler

ADC DL, 0 adds CF to DL, turning '0' into '1' when the bit was 1, with no jump needed.

Common mistakes

Using AND to set a bit (it can only clear) or OR to clear one (it can only set).
Using SHR on a negative number to divide it: use SAR, which keeps the sign.
Expecting SHL AX, 3 to work on a real 8086: use MOV CL, 3 and SHL AX, CL.
Clearing bit 5 of a character that is not a letter: check that it is between 'a' and 'z' first.

Practice problems for this lesson

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Upper and lower case

Shifts and rotates

Quick check

Practice problems for this lesson

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