Memories, SAP-1

• Read-Only Memory
  • diode ROM
  • ROM, PROM, EPROM, EEPROM
• Random Access Memory
  • static
  • dynamic
  • memory cycle: { (bar(CE)) }, { (bar(WE)) }
  SAP-1
  • architecture
  • instruction set

Diode ROM

• rows connected to the output of a decoder: only the selected row is high
• columns connected to the outputs of the RAM
• diodes from rows to columns raise the appropriate column (for a 1 bit), or do not raise it (a 0 bit)
• addressing with a demultiplexer (demux) that raises the correct row

ROM variants

• Read-only Memory (ROM): manufactured in bulk
• Programmable Read-only Memory (PROM): can be programmed once
• Erasable Programmable Read-only Memory (EPROM): a PROM that can be programmed, then erased with UV light, then programmed again
• Electrically Erasable Programmable Read-only Memory (EEPROM): a PROM that can be programmed, then erased with electrical signals, then programmed again (slower than RAM, limited number of reprogrammings)

RAM variants

• core memory
• static RAM: a collection of D flip-flops
• volatile RAM: a capacitor controlled by a gate. The capacitor leaks charge, so the memory must be refreshed every few milliseconds. The refreshing is done by on-chip circuitry, but cannot proceed in parallel with memory accesses

RAM read cycle

• raise (bar(WE))
• place address on address bus
• lower (bar(CE))
• after access time, read data on data bus

RAM write cycle

• lower (bar(WE))
• place address on address bus
• place data on data bus
• lower (bar(CE)) , hold it low for hold time

SAP-1

• Simple-As-Possible
• one output device (8 LEDs)
• 16 bytes of memory, read-only
• 5 instructions: 3 with 1 operand, 2 with implicit operands
• accumulator architecture: accumulator, out register, and, architecturally invisible, B register, MAR, and IR

SAP-1 Architecture

• 8-bit "W" bus
• 4-bit program counter only counts up (starts execution at 0)
• 4-bit Memory Address Register (MAR)
• 16x8-bit memory
• 8-bit instruction register (IR)
• 6-cycle controller with 12-bit microinstruction word
• 8-bit accumulator
• 8-bit B register
• 8-bit adder-subtractor
• 8-bit output register

SAP-1 Instructions