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254 Chapter 9 Implementation of C Procedures

9,1 Global and Local Variables

This section shows how a variable is allocated and accessed in C. The first part of this
section shows how to write a constant into a variable, which highlights the allocation
mechanism, and the addressing mode and instruction data width used with each variable.
The shorter second part describes what is done before the main procedure is executed, and
a short third part discusses access to I/O ports, which are treated as global variables in C,
In the first part of this section, we will use a short procedure main as an example
that we briefly show first. The names of the variables are abbreviations of their
characteristics; guc represents a global unsigned char, and gsi represents a global
signed int. The C program in Figure 9.la, which writes some constants into these
variables, is compiled into the assembly-language program in Figure 9.1b. Global
variables, which are declared outside a procedure, can be allocated by the compiler by the
equivalent of assembly-language ORG statement and DS statements. For instance, a
possible global declaration for Figure 9.1b is shown in Figure 9.1c.

unsigned char guc; int gsi;
roain(){ char Isc; unsigned int lui;
guc = 0; gsi = 5; Isc = 0; lui = 7;
}
a. A C Program

4: main(){ char Isc; unsigned int lui;
0000095B 1B9D LEAS ~3,SP
5s guc = 0;
0000095D 790800 CLR $0800
6: gsi = 5;
00000960 C605 LDAB #5
00000962 87 CLRA
00000963 7C0801 STD $0801
7: Isc = 0;
00000966 6A82 STAA 2,SP
8: lui = 7;
00000968 C607 LDAB #7
0000096A 6C80 STD 0,SP
10: }
0000096C 1B83 LEAS 3,SP
0000096E 3D RTS
b. Assembly Language Developed From Part (a)
ORG 2048 ; put global data at the beginning of RAM ($800)
guc: DS. B 1 ; allocate a byte for scalar char variable guc
gsi: DS. W 1 ; allocate two bytes for scalar int variable gsi
c. Declarations for Part (b)
Figure 9.1. A Simple Program