EE180: Microprocessor Systems Lecture Notes 6

Procedures – defined by PROC and ENDP directives. This is called by a CALL instruction and returns to the calling program by executing a RET instruction

Format of PROC and ENDP directives
a.    For PROC directive        Name    PROC    type
b.    For ENDP directive        Name    ENDP

Format of a Procedure
a.    Prefaced by explanatory comments
b.    Save and restore all the registers
c.    Must end with a RET instruction

For Calling a procedure, we use CALL instruction (format: CALL    address)
For returning to a calling program, we use RET Instruction (format:    RET)

Passing Parameters – procedures communicate by sending data items back and forth.

Input and Output parameter – is defines as the data items

Methods for Passing Parameters
a.    Passing Parameters in Register – used when passing a small number of parameters
b.    Passing Input Parameters by using the Stack – use the BP register to access the parameters in the stack

Arithmetic Numbers
a.    Fixed-point number – decimal point is fixed in one position (whole number).

This can be:
1.    Unsigned – represents only positive values and zero (0 to 255 for byte)
2.    Signed – represents positive and negative values and zero (-128 to 127 for byte)

b.    Floating-point number (represent values that have a fractional components)

Main processor – work with fixed-point numbers only
Math co-processor – works with floating-point numbers

Arithmetic Operations
a.    Add x1, x2 (add x2 and x1, the sum is stored at x1)
b.    Sub x1, x2 (subtract x2 from x1 [x1-x2], difference is stored at x1)
c.    MUL (for unsigned) and IMUL (for signed) – assumes that the second operand is in AL or AX
Format:        MUL    x1 ; assumes x2 is at AX or AL
d.    DIV (for unsigned) and IDIV (for sgined) - assumes that the second operand is in AL or AX

We use CBW for adding/subtracting a word and a byte specifically for SIGNED word
We use ADC for accounting the carry from the previous addition (add with carry)
We use SBB for accounting the borrow from the precious subtraction

Decimal Numbers – store values in base 10.

Two ways of storing decimal values
a.    Unpacked decimal numbers – each decimal digit is stored in its own byte. We usually use AAA or ASCII adjust for addition
b.    Packed decimal numbers – two decimal digits are stored in each byte. We use DAA or decimal adjust for addition

Function Calls – the services provided by DOS interrupt number 21h

DOS Function calls to do I/O
Handle	Use	Default Device
0000h	Standard input	Keyboard
0001h	Standard output	Monitor
0002h	Standard error	Monitor
0003h	Standard auxiliary	First Serial Device
0004h	Standard printer	Printer

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