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4 Using ST20-C1 instructions

ldc 0;

ldl Xlo; ldl Ylo; addc; stl Zlo;

ldl Xhi; ldl Yhi; addc; stl Zhi

The subscripts ‘lo’ and ‘hi’, used here and in subsequent text, specify the least and

most significant word respectively of the double word variable with which they are

associated.

Subtraction of two double length values,

Y from X giving Z, without overflow signalling

is compiled as

ldc 0;

ldl Xlo; ldl Ylo; subc; stl Zlo;

ldl Xhi; ldl Yhi; subc; stl Zhi

Overflow signalling for signed arithmetic may be added by performing an extra

addc or

subc to produce a final w ord which contains only a sign (0 for positive or -1 for

negative) unless an overflow has occurred. For example, the following code could be

used to perform double length signed addition with overflow signalling:

clear carry, overflow and underflow status bits

ld Xlo; ld Ylo; addc; st Zlo;

ld Xhi; ld Yhi; addc; st Zhi;

ldc 0; dup; addc;

dup; adc #7ffffff;

- overflows if and only if carry word > 0

rev; adc #8000001;

- underflows if and only if carry word < -1

Multiple length multiplication

The

umac instruction multiplies two single word unsigned operands in Areg and Breg,

and adds the single word carry operand in Creg to form a double length unsigned

result. The more significant (carr y) word of the result is left in Breg, the less significant

in Areg. No overflow can be signalled by this instruction.

Multiplication of a single length unsigned value

X by a double length unsigned value Y

(leaving the ‘carry’ in Areg) can be performed by:

ldc 0;

ldl X; ldl Ylo; umac; stl Zlo;

ldl X; ldl Yhi; umac; stl Zhi

Double length unsigned multiplication is more complex. The product of two unsigned

double length words

X and Y can be expressed as:

X* Y= (Xhi*2

32 +X

lo)*(Yhi*2

32 +Y

lo)

=(Xhi*Yhi)*2

64 +(X

hi*Ylo +Xlo*Yhi)*2

32 +(X

lo*Ylo)

This can be coded as follows:

ldc 0;

ldl Xlo; ldl Ylo; umac; stl Z0

ldl Xlo; ldl Yhi; umac; rev; stl Z2

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