W79E201 Datasheet, PDF (32/87 Pages) Winbond – 8-BIT MICROCONTROLLER

W79E201
8. INSTRUCTION
The W79E201 executes all the instructions of the standard 8032 family. The operation of these
instructions, their effect on the flag bits and the status bits is exactly the same. However, timing of
these instructions is different. The reason for this is two fold. Firstly, in the W79E201, each machine
cycle consists of 4 clock periods, while in the standard 8032 it consists of 12 clock periods. Also, in
the W79E201 there is only one fetch per machine cycle i.e. 4 clocks per fetch, while in the standard
8032 there can be two fetches per machine cycle, which works out to 6 clocks per fetch.
The advantage the W79E201 has is that since there is only one fetch per machine cycle, the number
of machine cycles in most cases is equal to the number of operands that the instruction has. In case
of jumps and calls there will be an additional cycle that will be needed to calculate the new address.
But overall the W79E201 reduces the number of dummy fetches and wasted cycles, thereby
improving efficiency as compared to the standard 8032.
8.1 Instruction Timing
The instruction timing for the W79E201 is an important aspect, especially for those users who wish to
use software instructions to generate timing delays. Also, it provides the user with an insight into the
timing differences between the W79E201 and the standard 8032. In the W79E201 each machine
cycle is four clock periods long. Each clock period is designated a state. Thus each machine cycle is
made up of four states, C1, C2 C3 and C4, in that order. Due to the reduced time for each instruction
execution, both the clock edges are used for internal timing. Hence it is important that the duty cycle of
the clock be as close to 50% as possible to avoid timing conflicts. As mentioned earlier, the W79E201
does one op-code fetch per machine cycle. Therefore, in most of the instructions, the number of
machine cycles needed to execute the instruction is equal to the number of bytes in the instruction. Of
the 256 available op-codes, 128 of them are single cycle instructions. Thus more than half of all op-
codes in the W79E201 are executed in just four clock periods. Most of the two-cycle instructions are
those that have two byte instruction codes. However there are some instructions that have only one
byte instructions, yet they are two cycle instructions. One instruction which is of importance is the
MOVX instruction. In the standard 8032, the MOVX instruction is always two machine cycles long.
However in the W79E201, the user has a facility to stretch the duration of this instruction from 2
machine cycles to 9 machine cycles. The RD and WR strobe lines are also proportionately
elongated. This gives the user flexibility in accessing both fast and slow peripherals without the use of
external circuitry and with minimum software overhead. The rest of the instructions are either three,
four or five machine cycle instructions. Note that in the W79E201, based on the number of machine
cycles, there are five different types, while in the standard 8032 there are only three. However, in the
W79E201 each machine cycle is made of only 4 clock periods compared to the 12 clock periods for
the standard 8032. Therefore, even though the number of categories has increased, each instruction
is at least 1.5 to 3 times faster than the standard 8032 in terms of clock periods.
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