Page 132 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

PROBLEMS 109

PROBLEMS

1. Suppose that the ORG statement is removed from the assembler. How can such
statements be handled by other directives, and what assumptions have to be made to
make it possible to completely replace the ORG statement?

2 . Although the assembler has an ALIGN, an EVEN, and a LONGEVEN assembler
directive, the ORG directive can align the location counter to an integer multiple of a
constant. Write such an ORG directive that aligns the next location to the next multiple
of four. Do not move the location counter if it is already aligned (low two bits are zero).

3 . An I/O port at location $cb is to be loaded with a constant. If bit 2 is asserted,
input hardware is activated; if bit 3 is asserted, output hardware is activated; if bit 5 is
asserted, input hardware can cause an interrupt; and if bit 7 is asserted, output hardware
can cause an interrupt. Write EQU directives that define four constants that can be ORed
together to generate a constant to be stored into the memory location or can be used with
BSET, BCLR, BRSET or BRCLR instructions. The constant ION turns on the input,
OON truns on the output, I INT enables input interrupts, and OINT enables output
interrupts. Write a MOVE instruction that turns on the input and output hardware and
enables input interrupts. Write an instruction that subsequently enables output interrupts
and another that disables output interrupts, neither of which change any other bits in
location $cb except bit 7. Comment on the use of symbols to improve clarity.

4 . A two-dimensional array of 8-bit elements is to have R rows and C columns. Write
EQU statements to define R to 3 and C to 4. Write a DC. B statement to allocate enough
storage for the array for any R and C. Write a shortest program segment that reads the
byte at row i column j into accumulator B, assuming rows and columns are numbered
starting with zero, and elements of a row are stored in consecutive memory locations.

5 . A vector of four 16-bit constants is to be initialized after location RATES, each of
which are calculated as 8,000,000 divided by 16 times the desired rate. For example, to
get a rate of 9600, put 52 into the element. The first element is to have a rate of 9600;
the next, of 1200; the next, of 300; and the last of 110. Write this DC directive.

6 . A vector of eight 8-bit constants is to be initialized after location PTRNA, each of
which is a bit pattern displayed on consecutive lines on a screen to draw a letter A. For
instance, the top row of eight bits will be $10, the next row will be $28, and so on.
Write this DC directive.

7 . Write a directive to clear all bytes from the current location counter to the location
whose four low address bits are zero.

8 . Write a directive to fill all bytes from the current location counter to location
$FFF6 with value $FF.