Question

Write a program to convert a series of packed BCD numbers to ASCII. Assume that the packed BCD is located in ROM locations starting at \(\$ 700\). Place the ASCII codes in RAM locations starting at \(\$ 40\). MYDATA: \(\quad .\) ORG \(\$ 700\) \(. \mathrm{DB} \$ 76, \$ 87, \$ 98, \$ 43\)

Short answer

The program initializes pointers for ROM (0x700) and RAM (0x40), converts each BCD digit to ASCII by adding 0x30 to each digit, and stores the ASCII characters into RAM sequentially.

Step-by-step solution

Understanding the Exercise

BCD (Binary Coded Decimal) represents each decimal digit in binary form, usually within a byte. Since it's packed, each byte contains two digits: the higher order 4 bits represent the tens digit and the lower order 4 bits represent the units digit. The task is to read these packed BCD numbers from ROM and convert them to their ASCII equivalent, which involves adding the ASCII value for '0' (48 in decimal or 0x30 in hexadecimal) to the BCD values to get the corresponding ASCII value for each digit, then storing these ASCII codes in RAM.

Initialize Pointers

Set up the starting address pointers for ROM and RAM. The starting ROM address is given as 0x700 and the RAM starting address for the ASCII codes is given as 0x40.

Convert BCD to ASCII and Store

Create a loop that will repeatedly load each BCD value from the ROM, convert both the tens and units digits to ASCII by separating them and adding 0x30 to each, and then store the two ASCII characters into consecutive RAM locations. This conversion takes advantage of the fact that the ASCII values for the numbers 0-9 are contiguous and start at 0x30 for the character '0'.

End of the Program

Loop until all packed BCD values are processed. Implement a check to end the loop after converting the last BCD value at ROM location 0x703.

Key concepts

Assembly Language Programming

Assembly language programming is considered a step above machine language in that it uses symbolic representations of the actual machine instructions. This makes it somewhat easier to read and write programs, as compared to writing raw binary code. To understand how a program converts BCD to ASCII in assembly, one must be familiar with the architecture of the microprocessor they are working with. This includes knowledge of the registers, the role of the accumulator, the instruction set, and how data moves between memory and the processor.

For the given exercise, handling packed BCDs requires understanding of bitwise operations, since one byte contains two decimal digits. The programmer must be able to isolate each digit and manipulate them individually. Operations typically employed in such tasks include masking, shifting, and adding offset values (in this case, 0x30 to convert to ASCII). Each of these operations corresponds to opcodes in assembly language that the CPU can execute. A mastery of assembly allows for highly optimized code, which is crucial in systems with limited resources, like embedded systems.

Embedded Systems

Embedded systems are specialized computing systems that are dedicated to performing specific functions and are often part of a larger mechanical or electrical system. Think of them as the 'brains' of everyday devices like digital watches, cars, and microwaves. Programming for embedded systems can involve using assembly language for direct hardware manipulation and performance optimizations.

In our BCD to ASCII conversion exercise, an embedded system could ideally be where this program would operate. For example, it may be part of a larger control system which reads sensor data encoded in BCD and presents it in a human-readable format. In such systems, ROM (Read-Only Memory) is used to store immutable data like our BCD values, while RAM (Random Access Memory) is used for data that changes over time, such as the ASCII values being produced in real-time. Efficiency is key in these systems; hence, the need for assembly programming which allows detailed management of system resources.

Microcontroller Programming

Microcontroller programming involves writing software to control small computing devices that are embedded within electronic systems. Microcontrollers are less powerful than full-fledged CPUs but are much more cost-effective and power-efficient for simple, dedicated tasks. Languages used for programming microcontrollers range from high-level languages like C to low-level languages like assembly.

The exercise we have highlights a typical application in microcontroller programming - data manipulation. Microcontrollers in an embedded system may be tasked with reading sensor values, performing computations, and displaying results. Since the resources are constrained, the solution often lies in efficient programming practices. In a pack of BCD numbers conversion, careful incrementing of addresses in ROM, precise handling of the BCD to ASCII conversion, and proper storage in RAM showcase the finesse that is demanded in microcontroller programming. The added efficiency of assembly language in such programming contexts cannot be overstated, for it aligns closely with the hardware's operational model.