Computer Architecture Lecture 6 by Engineer A. Lecturer Aymen Hasan AlAwady 1/12/2013 University of Kufa - Informatics Center for Research and Rehabilitation 1 1

Bus Timing. Cont. Review Example: MVI A,32H (means load 32h in the register (A)) 2000H 3EH 2001H 32H 1.Illustrate the bus timing ? 2.Calculate the time required to execute the opcode and memory read if the clock frequency is 2MHz

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Bus Timing. Cont. Clock frequency= 2 MHz T-state= clock period (1/f) = 0.5 µs Execution time of Opcode fetch: (4T)= 2 µs Execution time of Memory read: (3T)= 1.5 µs So, Execution time of Instruction: (7T)= 3.5 µs 4

1. Memory interfacing There needs to be a lot of interaction between the microprocessor and the memory for the exchange of information during program execution. Memory has its requirements on control signals and their timing. The microprocessor has its requirements as well. (Read and write from and on it) The interfacing operation is simply the process of designing a circuit to match these requirements. 5

1.2 Memory structure & its requirements The way of interfacing the above two chips to the microprocessor is the same. However, the ROM does not have a WR signal which is for input buffer and RD for output buffer. EPROM 4096×8 Address Lines Date Lines CS RD Output Buffer ROM R/W Memory 2048×8 Address Lines Data Lines CS RD Output Buffer RAM WR Input Buffer Data Lines A10 A0 A11 A0 Internal decoder 6

1.3 Interfacing Memory Accessing memory can be summarized into the following three steps: Select the chip. Identify the memory register. Enable the appropriate buffer. Translating this to microprocessor domain: The microprocessor places a 16-bit address on the address bus. Part of the address bus will select the chip and the other part will go through the address decoder to select the register. The signals IO/M and RD combined indicate that a memory read operation is in progress (MEMR ) and it can be used to enable the RD line on the memory chip. 7

Timing of memory read cycle 8

Timing of memory write cycle 9

1.4 Address decoding The result of ‘address decoding’ is the identification of a register for a given address. A large part of the address bus is usually connected directly to the address inputs of the memory chip. This portion is decoded internally within the chip. What concerns us is the other part that must be decoded externally to select the chip. This can be done either using logic gates or a decoder. 10

Putting all of the concepts together: Back to the Overall Picture A15-A8 Latch AD7-AD0 D 7 - D 0 A 7 - A ALE IO/M RD WR 1K Byte Memory Chip WR RD CS A 9 - A 0 A 15 - A 10 Chip Selection Circuit 11

A13 A12 A14 A15 Address Decoding using NAND A13 A12 A14 E1 E2 E3 O 3-to-8 Decoder A15 Address Decoding using 3-to-8 Decoder 12 Address Decoding types

Interrupt signals An interrupt is a hardware-initiated subroutine CALL. When interrupt pin is activated, an ISR will be called, interrupting the program that is currently executing. ISR: interrupt Service Register, a type of register in a Programmable Interrupt Controller Pin Subroutine Location TRAP0024 RST C RST RST C INTR* Note: * the address of the ISR is determined by the external hardware.

Interrupt signals INTR input is enabled when EI (Embedded instruction) is executed. The status of the RST 7.5, RST 6.5 and RST 5.5 pins are determined by both EI instruction and the condition of the mask bits in the interrupt mask register.

15 Interrupt Vectors

16 A circuit that causes an RST4 instruction (E7) to be executed in response to INTR. When INTR is asserted, 8085 response with INTA pulse. During INTA pulse, 8085 expect to see an instruction applied to its data bus.

RESET signal Following are the two kind of RESET signals: RESET IN: an active low input signal, Program Counter (PC) will be set to 0 and thus MPU will reset. RESET OUT: an output reset signal to indicate that the μp was reset (i.e. RESET IN=0). It also used to reset external devices.

18 RESET signal

Direct Memory Access (DMA) DMA is an IO technique where external IO device requests the use of the MPU buses. Allows external IO devices to gain high speed access to the memory. Example of IO devices that use DMA: disk memory system. HOLD and HLDA are used for DMA. If HOLD=1, 8085 will place it address, data and control pins at their high-impedance. A DMA acknowledgement is signaled by HLDA=1.

Reading assignment (Report) 1.Topic ( Challenges in memory interfacing ) 2.Write as minimum 3 pages in clear English and give your conclusions and ideas about any potential solutions. 3.Font 12 pt Times New Roman. 4.DO NOT COPY AND PASTE (plagiarism). 5.List your references. 6.Write down your name and group clearly. 7.Submit your paper on in MS Word doc(x) 8.Dead-line is the end Monday 9 / 12/