ST9 datasheet(36/146 Pages) STMICROELECTRONICS

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ST9 USER GUIDE

push r6

push (R120)

push #80

pushw RR100

pushw #1500

Pushing an immediate value is especially useful when you are programming in C.

A special push instruction is Push Effective Address. This instruction does not push the data

itself, but the memory address of the data. For example:

pea 5(rr2)

This takes the contents of rr2, adds 5 and pushes the result onto the stack. This is widely used

in C programming.

3.3.1.4 Multiply and Divide

The multiply instruction takes two byte operands and provides a word result. All numbers are

treated as unsigned numbers (operands 0 to 255, result 0 to 65535). Though both operands

are bytes, the first one must address a word to receive the result. The first operand should

then reside in the low byte of the word, and the high byte, not used in the operation, will be

overwritten. The flag register is affected but the state of the flags after the operation is mean-

ingless.

To multiply a signed number (operands -128 to 127) by an unsigned number (operands 0 to

255) with a result in the range of -32768 to 32767, refer to the example given below:

r1,#signed_data

r4,#unsigned_data

btjt

r1.7,neg

mul

rr0,r4

; rr0 = r1*r4, with r1 value a positive signed

; number

end

neg:

mul

rr0,r4

; rr0 = r1*r4, with r1 value a negative signed

; number

sub

r0,r4

; rr0=rr0-100h*r4

end

With the signed operand equal to 226=0E2h (means -30 for signed data) and the second un-

signed operand equal to 0Fh (+15) the result will be -450 (0FE3Eh).

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