PART 2 ORGANIZATION OF THE ALTAIR 8800¶
A block diagram showing the organization of the ALTAIR 8800 is shown in Figure 2-1. It is not necessary to understand the detailed electronic operation of each part of the computer to make effective use of the machine. However, a general understanding of each of the various operating sections is important.
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(Click CPU Diagram for a larger image.)
A. CENTRAL PROCESSING UNIT (CPU)¶
The Central Processing Unit (CPU) performs all arithmetic calculations, makes all logical decisions, controls access to the computer by input and output devices, stores and retrieves data from the memory, and coordinates the orderly execution of a program. The CPU is quite literally the heart of the computer.
Of course it is important to remember that the CPU is only as intelligent as the programmer, for the CPU must be instructed in precise terms just how to perform a particular operation. But since the CPU in the ALTAIR 8800 can execute a complete instruction cycle in only 2 microseconds*, the computer can solve a highly complex problem in an incredibly brief time. In fact, the ALTAIR 8800 can execute a six instruction addition program approximately 30,000 times in one second.
The compact size and economy of the ALTAIR 8800 is in large part due to the CPU. Thanks to large scale integrated circuit techniques (LSI), the CPU used in the ALTAIR 8800 is fabricated on a tiny silicon chip having a surface area of only a fraction of an inch. This chip, the Intel 8080, is installed in a protective dual-in-line mounting package having 40 pins.
The CPU is by far the most complex portion of the ALTAIR 8800. A complete block diagram of the CPU is shown in Figure 2-2, and while it is not necessary to possess a detailed understanding of this diagram it is important to understand the role of some of the CPU’s more important systems. The interrelationship of each of these systems and their contribution to the operation of the CPU will then become more obvious.
TIMING AND CONTROL–The timing and Control System receives timing signals from the clock and distributes them to the appropriate portions of the CPU in order to insure coordinated instruction execution. The Timing and Control System also activates several front panel status indicators (HOLD, WAIT, INTE, STACK, OUT, IN, INP, MI MENR, HLTA, WO, INT).
A microsecond is one millionth of a second.
INSTRUCTION REGISTER–Binary machine language instructions are temporarily stored in the Instruction Register for decoding and execution by the CPU.
ARITHMETIC–The Arithmetic System performs both binary and decimal arithmetic. All arithmetic operations are performed by addition. Multiplication is implemented by repetitive addition. Subtraction and division are implemented by inverse addition.
WORKING REGISTERS–The CPU contains seven 8-bit Working Registers. The most important of these is the Accumulator, the register into which the results of many operations are eventually loaded. In addition to acting as a primary storage point for results of many program operations, numerous arithmetic and logical operations can be performed with the Accumulator and any specified register or memory address,
The six remaining registers, which are arranged in pairs to permit 16-bit operation when necessary, are “scratchpad” registers. This simply means they are used to store temporary data or addresses on a regular basis and are available for numerous program operations.
Figure 2-3 shows the arrangement and classification of the seven Working Registers. The additional register adjacent to the Accumulator, the Status Bit Register, is a special purpose register used to store the status of certain operations.
Register Pair B |
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Register Pair D |
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Register Pair H |
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Register Pair PSW |
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* Status Bit Register (See Text)
STATUS BIT REGISTER–The Status Bit Register is a special purpose register which stores the status of five conditions which may or may not be affected by the result of a data operation. This register contains 8-bit positions, but only 5-bits are used to store the status information. The five status bits are:
Carry Bit–This bit is set to 1 if a carry has occurred. The Carry Bit is usually affected by such operations as addition, subtraction, rotation, and some logical decisions. The bit is set to 0 if no carry occurs.
Auxiliary Carry Bit–If set to 1, this bit indicates a carry out of bit 3 of a result. 0 indicates no carry. This status bit is affected by only one instruction (DAA).
Sign Bit–This bit is set to show the sign of a result. If set to 1, the result is minus; if set to 0 the result is plus. The Sign Bit reflects the condition of the most significant bit in the result (bit 7). This is because an 8-bit byte can contain up to the decimal equivalent of from -128 to +127 if the most significant bit is used to indicate the polarity of the result. 23
Zero Bit–This bit is set to 1 if the result of certain instructions is zero and reset to 0 if the result is greater than zero.
Parity Bit–Certain operations check the parity of the result. Parity indicates the odd or even status of the 1 bits in the result. Thus if there is an even number of 1 bits, the Parity Bit is set to 1, and if there is an odd number of 1 bits, the Parity Bit is set to 0.
PROGRAM COUNTER–The Program Counter is a special 16-bit register which stores the address of the next program step to be executed. The Program Counter is automatically advanced to the next sequential program address upon completion of a step execution. Sometimes called the P-Counter, the Program Counter is directly accessible to the programmer via machine language instructions which implement JUMP, CALL, and RETURN instructions.
STACK POINTER–The Stack Pointer is another special 16-bit register. A section of memory reserved for the temporary storage of data or addresses is called the stack.
Data can be pushed onto the stack for temporary storage and popped out of the stack via several instructions.
The Stack Pointer is used to store the contents of the Program Counter during the execution of subroutines. A RETURN instruction transfers the contents of the Stack Pointer to the Program Counter and sequential execution of the main program continues. The programmer selects the location of the stack in memory by loading the Stack Pointer with the desired memory address via a special instrution (LXI).
The interrelationship of the Working Registers, Program Counter, Stack Pointer, Arithmetic System, Instruction Register, and Timing and Control System should now be more meaningful. The Working Registers incorporate six scratchpad registers and an Accumulator into which numerous operation results are temporarily stored. The Program Counter causes sequential execution of a program by keeping track of the memory address of the next instruction to be executed. The Timing and Control System supplies timing pulses which coordinate orderly program execution. The Stack Pointer is used for temporary storage of the data contained in any register pair. The Stack Pointer also saves the address in the Program Counter for retrieval after a subroutine has been executed. All these operations combine to provide an enormously flexible and versatile CPU.
B. MEMORY¶
Though the Working Registers, Program Counter, and Stack Pointer certainly perform memory roles, the CPU does not contain memory as it is normally defined in a computer application. The primary memory in a computer is external to the CPU.
Simple programs can be implemented with a few dozen words of memory or even less, but more complex applications such as video processing require more memory. The ALTAIR 8800 is expandable to 65,536 8-bit words of memory.
Access to the memory is always controlled by the CPU.* 16 address lines called the Address Bus connect the CPU to the Memory. These lines permit the CPU to input or output data to or from any memory address. The addresses are specified by two 8-bit bytes. The CPU processes each address as two sequential (serial) cycles, each containing 8-parallel bits. Data stored in the Memory is exchanged between the Memory and CPU via 8 data lines called the Data Bus. This interconnection format permits parallel operation. Thus, when data is read from or written to Memory by the CPU, it is transmitted as a complete 8-bit word.
The basic Memory in the ALTAIR 8800 contains up to eight 256 x 4 bit random access memories (RAMS). However, any conventional memory can be used in the computer if input loading on the buss does not exceed 50 TTL loads and if the buss is driven by standard TTL loads.
An exception to this is when the computer is connected to a Direct Memory Access Controller. DMA takes control of the address and data lines from the CPU for direct transfers of blocks of data. These transfers can take place internally (from one memory location to another) or externally (from memory to an external device).
C. CLOCK¶
Orderly execution of a program by the CPU is controlled by a 2-MHz crystal controlled clock. Crystal control is used to permit the clock to operate at the maximum permissible CPU speed. A clock without crystal regulation might occassionally speed up beyond the CPU’s capability and program execution errors would result.
D. INPUT/OUTPUT¶
The ALTAIR 8800 can be interfaced with a great many external devices. Generally, these devices provide input information to the computer and accept output information from the computer. The CPU monitors the status of program execution and Input/Output devices and provides the necessary signals for servicing external devices. The programmer can instruct the CPU to either ignore or respond to interrupt signals provided by an external device. These interrupt signals, when accepted by the CPU, cause the program execution to be temporarily halted while the external device is serviced by the computer. When the external device has been serviced, the program resumes normal execution. The ALTAIR 8800 will service up to 256 Input and 256 Output devices.
This concludes the description of the organization of the ALTAIR 8800. The overall operation of the computer as a powerful and efficient data processing system will become more apparent in Part 3, a discussion of the operation of the ALTAIR 8800.
