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The Pep/7 Computer Simulation (also known as Pep7) is a virtual machine that is used by professors of schools and colleges to explain and simplify the lowest level of computing (machine language). True to its name, Pep/7 has seven registers and four status bits.[1][2]

The Pep/7 is a direct descendant of the Pep/6 virtual machine,[3] the only major difference is that it has been expanded from 4KiB to 32KiB of system RAM. The machine code format remains the same.


Pep/7 registers
15 ... 07 ... 00 (bit position)
Data registers
A Accumulator
X Index
Base Register
B Base
Stack pointer
SP Stack Pointer
Program counter
PC Program Counter
Condition Code Register
  03 02 01 00 (bit position)
  N Z V C Flags

Address Bus[edit]

The Pep/7 uses a 16-bit address bus, capable of addressing 65,536 bytes

Internal Registers[edit]

The CPU has two 16-bit general purpose registers, A and X, a base pointer register B, a stack pointer SP, and an instruction pointer PC. There is also a hidden register IR that is 8-bits wide and holds the currently executing opcode. It is not normally accessible by the programmer.

Instruction Set[edit]

The Pep/7's instruction set is simplistic, with only 32 possible opcodes. The instruction specifier consists of the opcode of 5 bits, a register selector (A or X) of 1 bit, and a memory access mode of 2 bits, with the opcode occupying the top 5 bits of a byte, the register selector in bit 2 and the address mode in bits 1 and 0.

Opcode List
Mnemonic Opcode Flags Affected Addressing modes available
STOP 00000 - -
LOAD r 00001 NZ idsx
STORE r 00010 - dsx
ADD r 00011 NZVC idsx
SUB r 00100 NZVC idsx
AND r 00101 NZ idsx
OR r 00110 NZ idsx
NOT r 00111 NZ -
ASL r 01000 NZVC -
ASR r 01001 NZC -
LDBYT r 01010 NZ idsx
STBYT r 01011 - dsx
LOADB 01100 NZ idsx
ADDSP 01101 NZVC i
BR 01110 - ix
BRLE 01111 - ix
BRLT 10000 - ix
BREQ 10001 - ix
BRNE 10010 - ix
BRGE 10011 - ix
BRGT 10100 - ix
BRV 10101 - ix
BRC 10110 - ix
COMP r 10111 NZVC idsx
JSR 11000 - ix
RTS 11001 - -
RTI 11010 - -
CHARI 11011 - dsx
CHARO 11100 - idsx
DECI 11101 NZV dsx
DECO 11110 - idsx
HEXO 11111 - idsx

Instruction Set Details[edit]

Four addressing modes are available. These are:

  • i, immediate mode, where the operand is stored directly following the opcode in memory.
  • d, direct mode, where the immediate operand is a pointer to data in memory.
  • s, stack relative mode, where the immediate operand is added to the SP register to generate a memory address.
  • x, indexed mode, where the B register is added to the X register to obtain a memory address.

These memory modes are specified by bits 0 and 1 of the opcode: 00=i, 01=d, 10=s, and 11=x.

External links[edit]


  1. Dale, Nell, and John Lewis. Computer Science Illuminated. 3rd ed. Sudbury, MA: Jones and Bartlett, 2007. Print.
  2. Westwood Public Schools[permanent dead link]
  3. Warford, J. Stanley. Computer Systems, 1st ed. Jones & Bartlett. Search this book on Logo.png

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