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STANDARD PROCEDURE WRITE

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This procedure's call allows us to output value of the variables of different types (INTEGER, CHAR, BOOLEAN) to display screen.

It is evident that this construction is in some parts similar to assign operator: both of them load the value of necessary variable.

The execution of WRITE construction consists from 2 steps:
get value of the variable to fixed register (R1 for "E97" and AX for Intel)
print this value using ROM subroutines.

Here we'll consider mainly the second part, because the first was under discussion in assignment.

"COMPAS" compiles WRITE construction to a call of standard output subroutine. In "E97" these subroutines are placed into ROM, and for Intel code - into Run-Time Library (RTL), which automatically adds to every executable program code.

Operator's structure

"E97" code bricks

Example of full operator in "E97" code

Intel code bricks

Example of full operator in Intel code

I called bricks the short pieces of code, that "COMPAS" generate as a part of operator. Compiler builds result machine code from the bricks.

Operator's structure

Examples of typical available in "COMPAS" constructions are exemplified below (they are identical for all types of variables):

WRITE(i);
WRITE(a[i]);

WRITELN(i);
WRITELN(a[i]);

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"E97" code bricks

Here is a short pieces of "E97" code for output values of variables, which "COMPAS" inserts as the second part of WRITE construction (the first part - read this value from memory):

for INTEGER type for CHAR type for BOOLEAN type

01D3
00E0 set buffer address to R3

9C0D
4068 call ROM subroutine 4068 9C0D
40BA call ROM subroutine 40BA 9C0D
40C4 call ROM subroutine 40C4

Note that for INTEGER type output is the most complex procedure. So "COMPAS" has to reserve the 6-byte buffer for converting integer binary value to string. A special memory region 00E0 - 00E5 is used for these purposes.

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Example of full operator in "E97" code

WRITELN(c); (here DE is CHAR variable c address)

Code Operation Pascal comments

81E1
00DE (DE)_b ==> R1 Get byte value of variable c

9C0D
40BA call 40BA Print CHAR value from R1

9C0D
40EC call 40EC To next display line

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Intel code bricks

Here is a short pieces of Intel code for output values of variables to display, which "COMPAS" inserts as a part of WRITE construction (the first part of the operator - get value from memory is not shown here):

for INTEGER type for CHAR type for BOOLEAN type

call 157 call subroutine
from RTL call 186 call subroutine
from RTL call 1B4 call subroutine
from RTL

You can note the difference only in subroutine address.

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Example of full operator in Intel code

WRITELN(i); (here 4FE is INTEGER variable i address)

Code Operation Pascal comments

A1 FE 04 mov ax,[4FE] Put variable i value into AX

E8 ?? ?? call 157 Print AX value

E8 ?? ?? call 191 To next display line

Note that the content of displacement in both call instructions (see the first column) depends from code position in memory (the address of RTL subroutine is always constant in "COMPAS"). So I wrote ?? ?? instead of concrete hex-digits in these positions.

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