Multiplication with LEA Using Non-Powers of Two
 
Are you impressed yet with all that LEA can do on the 386? Believe it or not,
one more feature still awaits us. LEA can actually perform a fast multiply of a
32-bit register by some values other than powers of two. You see, the same
32-bit register can be both base and index on the 386, and can be scaled as the
index while being used unchanged as the base. That means that you can, for
example, multiply EBX by 5 with:
 
lea ebx,[ebx+ebx*4]
 
Without LEA and scaling, multiplication of EBX by 5 would require either a
relatively slow MUL, along with a set-up instruction or two, or three separate
instructions along the lines of the following
 
mov edx,ebx
shl ebx,2
add ebx,edx
 
and would in either case require the destruction of the contents of another
register.
 
Multiplying a 32-bit value by a non-power-of-two multiplier in just 2 cycles is
a pretty neat trick, even though it works only on a 386 or 486.

; The precision Zen timer (PZTIMER.ASM)
;
; Uses the 8253 timer to time the performance of code that takes
; less than about 54 milliseconds to execute, with a resolution
; of better than 10 microseconds.
;
; By Michael Abrash
;
; Externally callable routines:
;
; ZTimerOn: Starts the Zen timer, with interrupts disabled.
;
; ZTimerOff: Stops the Zen timer, saves the timer count,
; times the overhead code, and restores interrupts to the
; state they were in when ZTimerOn was called.
;
; ZTimerReport: Prints the net time that passed between starting
; and stopping the timer.
;
; Note: If longer than about 54 ms passes between ZTimerOn and
; ZTimerOff calls, the timer turns over and the count is
; inaccurate. When this happens, an error message is displayed
; instead of a count. The long-period Zen timer should be used
; in such cases.
;
; Note: Interrupts *MUST* be left off between calls to ZTimerOn
; and ZTimerOff for accurate timing and for detection of
; timer overflow.
;
; Note: These routines can introduce slight inaccuracies into the
; system clock count for each code section timed even if
; timer 0 doesn’t overflow. If timer 0 does overflow, the
; system clock can become slow by virtually any amount of
; time, since the system clock can’t advance while the
; precison timer is timing. Consequently, it’s a good idea
; to reboot at the end of each timing session. (The
; battery-backed clock, if any, is not affected by the Zen
; timer.)
;
; All registers, and all flags except the interrupt flag, are
; preserved by all routines. Interrupts are enabled and then disabled
; by ZTimerOn, and are restored by ZTimerOff to the state they were
; in when ZTimerOn was called.
;
[...]