About Pep/10

• Pep/10 is the new architecture for upcoming Computer Systems, 6th edition (CS6E), with some major changes over Pep/9 and Computer Systems, 5th edition

  • Both have companion software to allow students to write & debug assembly language programs

  • CS6E will come with a new IDE running on Win/Mac/Linux/WASM, containing tools for Pep/10, Pep/9, and Pep/8; I can finally stop maintaining CS4/5E software

• 16-bit CISC ISA used to teach computer architecture & assembly language to undergraduates

  • Designed for easy hand translation from C to assembly

• Accumulator machine with 1 general purpose register and 5 special registers. See register descriptions in the Registers section

• Unary instructions do not access memory and only operate directly on registers

• Non-unary instructions contain a extra 16-bit operand, which is interpreted using one of 8 addressing modes

• With only 1 GP register, C operations like B, A = A, B need extra temporary variables in memory

• Uses memory-mapped IO

  • Read and write specific memory addresses for stdin, stdout rather than accessing files

Registers

Register	Short Name	Special or General?	Usage
Accumulator	A	General	Destination register for most instructions
Index	X	General-ish	Offsets for some addressing modes; alternative destination register
Stack Pointer	SP	Special	Contains the current stack pointer, implicitly modified by instructions CALL and RET
Program Counter	PC	Special	Contains the address of the next instruction to execute. Modified with control flow mnemonics like BR, CALL, and RET
Instruction Specifier	IS	Special	Contains the current opcode. Not accessible by the user
Operand Specifier	OS	Special	Contains the opcode's argument if it is non-unary. Not accessible by the user

Addressing Modes

Addressing Mode	aaa-field	a-field	Letters	Operand	Commonly used to Access...
Immediate	000	0	i	OS	Constants
Direct	001		d	Mem[OS]	Global variables
Indirect	010		n	Mem[Mem[OS]]	Items pointed to with a global pointer
Stack-relative	011		s	Mem[SP + OS]	Variables allocated on the stack
Stack-relative deferred	100		sf	Mem[Mem[SP + OS]]	Items pointed to with stack-allocated pointers
Indexed	101	1	x	Mem[OS + X]	Items in global arrays
Stack-Indexed	110		sx	Mem[SP + OS + X]	Fields in stack-allocated structs, items in stack-allocated arrays
Stack-deferred indexed	111		sfx	Mem[Mem[SP + OS] + X]	Fields in struct pointed to by a stack-allocated pointer