Intel 80C196NU, 8XC196NP manual Operating Modes, Saturation Mode, Multiply/Accumulate Example Code

8XC196NP, 80C196NU USER’S MANUAL

Table 3-1.		Multiply/Accumulate Example Code
Device		Instructions	Execution Time
8XC196NP	mul	temp,operand_2,operand_1	16 states		1280 ns
(25 MHz; 1 state time = 80 ns)	shll	temp,#1	8 states		640 ns
	add	out_l,temp_l	4 states		320 ns
	addc	out_h,temp_h	4 states		320 ns
			32 states total	2560 ns total
80C196NU	mul	08H,operand_2,operand_1†	16 states		640 ns
(50 MHz; 1 state time = 40 ns)
			16 states total		640 ns total

†Because bit 3 of the destination address (08H) is set, the 80C196NU clears the accumulator before adding the result of the current instruction to it. If bit 3 were clear (destination address 07H–00H), the 80C196NU would add the result of the current instruction to the existing value of the accumulator.

3.2OPERATING MODES

The accumulator has two operating modes that allow you to control the results of operations on signed numbers. These modes are called saturation mode and fractional mode.

3.2.1Saturation Mode

Saturation occurs when the result of two positive numbers generates a negative sign bit or the re- sult of two negative numbers generates a positive sign bit. Without saturation mode, an underflow or overflow occurs and the overflow (OVF) flag is set. Saturation mode prevents an underflow or overflow of the accumulated value. In saturation mode, the accumulator’s value is changed to 7FFFFFFFH for a positive saturation or 80000000H for a negative saturation and the sticky sat- uration (STSAT) flag is set. The following two examples illustrate the contents of the accumulator as a result of positive and negative saturation, respectively:

7FFFFFFFH = 0111 1111 1111 1111 1111 1111 1111 1111 = 231 – 1 = +2147483647

80000000H = 1000 0000 0000 0000 0000 0000 0000 0000 = –2147483648

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