Hardware manual

Microcomputer Technical Q&A
7
Q&A No.: QAH8S-006
Category: CPU
Subject: MAC Register
Question
Why is the MAC register 42 bits long?
Answer
The MAC register is used to store the value obtained by adding together the result of a
multiplication and the value in the register itself. The multiplication result is 32 bits (from a 16-bit
× 16-bit operation), and if the MAC register value is added as 32 bits, overflow may occur. To
prevent overflow, therefore, the 32 bits are increased by 10 bits, giving 42 bits (so that overflow
will not occur even if 2
10
= 1024 multiply-and-accumulate operations are performed).
Applicable Products
Applicability Series Applicability Series Applicability Series
Entire H8S Series Yes H8S/2655 H8S/2350
H8S/2355 H8S/2357 H8S/2345
H8S/2245 H8S/2148 H8S/2144
H8S/2138 H8S/2134 H8S/2128
H8S/2124 — — — —