1. Introducción a
microprocesadores VI
Memorias

2. Pin layout de un SRAM

3. Pins de datos
• Como la SRAM es una memoria de
lectura/escritura, los pins de datos son usados
para leer datos en el chip y ademas para
escribir informacion hacia el microprocesador
• Con esto en mente, se le llamaran pins de
entrada/salida (I/O).

4. Control pins
• 1 Write enable (WE) controls internal tri-state buffers to control the
• flow of data to write or read on the data pins. The line over the WE
• indicates that, to write data, the pin must be held low.
• 2 There are two chip selects, one shown as active low and one as
• active high. To enable the chip, both must have the correct voltage
• present. This provides a degree of flexibility to the system designer.
• If either is not required, it can simply be held down permanently to
• its appropriate voltage and then the other can then be used to
• control the operation of the chip.

5. Pin layout of a DRAM
• Row address strobe (RAS) and Column address strobe (CAS)
• At first glance, there does not seem to be enough address pins for the
• number of locations to be addressed. A0 to A9 is only 10 pins, which
• would suggest a total of 210 or 1024 locations. The trick here is to use
• the same pins twice and hence load in a total 220 or 1 048 576
• addresses.
• The sequence of events dictates that the RAS line is taken low and the
• bottom half of the address is loaded into the Rows – RAS then returns
• high. Then the CAS is taken low while the remainder of the address is
• fed into the columns. After this, the state of the write line (same as R/W)
• determines whether data is entering or leaving the DRAM.
• There is only a single bit stored in each address, the data entering via
• the data in (Din) pin and leaving via the data out (Dout) pin.

6. Some more memories that don’t fit
into the general pattern
• SIMMs
• Single in-line memory modules are a
collection of separate RAM chips
• that are mounted on a piece of board to make
installation quicker and
• easier. They are not actually a different type of
memory.

7. RAM Cards
• The problem with RAM memory is that it is volatile.
The information
• is lost as soon as the system is switched off. RAM cards
overcome this
• by providing the RAM chips with their own on-board
battery. In this
• way, the RAM card can be removed from the system
without losing the
• data. It is really a RAM pretending to be ROM. This
provides full speed
• operation and permanence during the 10-year life of
the battery.

8. Flash memory
• This is non-volatile RAM (NVRAM). In fact, it is
really a form of RAM
• with a battery installed to provide power
during shutdown periods.
• Rather like a single chip version of a RAM card.

9. Memory maps
• A microprocessor has a number of address lines that can be used to
• access RAM or ROM or other devices within the system. As we saw in
• the memory chips, the total memory addressable by a microprocessor
• is found by the formula 2n where n is the number of address lines. For
• example, an 8-bit microprocessor generally has 16 address lines and
• can access 216 or would have 65 536 or 64 bytes of memory. The
• Digital Alpha 21064 has 34-bit address lines giving 234 or a little over
• 17 Gbytes. This memory is shared between the RAMs, ROMs and
• other devices, including some for the microprocessor itself to use.
• The system designer has to decide in what way the available memory
• is to be used. Using the memory map of the 8-bit microprocessor as a
• simple example, we start off with a blank space as in Figure 6.21.
• When the microprocessor is first supplied with power it will
• immediately start following the first program provided. How does it

10. Mapeo de memoria

11. Mapeo de memoria

12. Mapeo de memoria
• know what program is first? The answer to this is that it goes to a predetermined
• address, which has been built into the microprocessor
• during manufacture.
• If we assume the startup address is FFFAH, then we must put some
• useful information at that address for it to follow. This implies that
• some ROM memory must be at the top end of the memory map. Most
• provide some extra fast programming ability and this usually requires
• that some RAM to be available at the bottom end of the memory
• map.
• There is nothing else that is allocated to any particular address so any
• other RAM and ROM memories can be placed at any position on
• the map. The map does not have to be full, indeed it seldom is. The
• balance between RAM and ROM depends on the purpose to which the
• system is to be put. A control system running a piece of manufacturing
• machinery is likely to be predominately ROM whereas a computer
• would need significant RAM. A simple memory map is shown in
• Figure 6.22.

13. Acomodando la memoria
• Sorting out the addresses
• This is just an exercise in hex numbers in which a ‘hex’ calculator will
• prove invaluable. Now, 1 kbyte of memory occupies 210 or 1024
• locations, which is 400H in hex. The first section of RAM extends
• from: start address + highest RAM address = 0000H to 03FFH so the
• highest address in the ‘Fast instructions’ section is 03FFH. The User
• RAM extends from 0400H to 3FFFH. How many kilobytes of memory
• is this?
• This is 15 kbytes of memory. This was found by subtracting 0400H
• from 4000H to give 3C00H and then dividing this result by 400H, the
• hex equivalent of 1 kbyte. The division is best done by calculator.
• Remember that the fact that the user-RAM ended at 3FFFH means that
• the total number of locations, including the first one in 4000H so it
• was more convenient to use the figure of 4000H straight from the
• memory map. This is sometimes a little difficult to fully come to terms
• with, but a cup of coffee and a slump in an armchair often helps.
• In a similar way, the video RAM that holds the information to be
• displayed on a monitor can be found by subtracting the lower address
• D800H from the higher address E000H to give 800H. Dividing by
• 400H indicates 2 kbytes of video RAM.

14. • 94