The CPU (Central Processing Unit) executes instructions. The instructions it supports are almost all documented in S/370 POPs (Principles of Operation, although other people abbreviate it other ways). S/370 supported a maximum of 2 CPUs, as I recall. The Hercules emulator knows how many virtual CPUs to provide by the NUMCPU configuration statement. The tutorial assumes you have configured Hercules to only provide one CPU, and will not deal with any implications of there being more than one virtual CPU. Back when MVS38j was current, most systems only had one CPU.

S/370 memory is Big Endian, up to a usual maximum of 16 MB. There are eight bits per byte, two bytes are called a halfword, four bytes are called a fullword, and eight bytes a doubleword. By convention, S/370 bits are numbered beginning at zero on the left. You will frequently see S/370 memory referred to as "main storage".

Registers come in several varieties in S/370: general purpose (32 bit integers, signed or unsigned depending on instruction), floating point (64 bits), and control (also 32 bits). The PSW (Program Status Word) maintains S/370 state information, and includes the memory address of the next instruction to be executed.

Channels are the medium by which the CPU passes I/O commands to a "control unit" which in turn passes commands to I/O devices. There are a wealth of different types of I/O devices supported by S/370, such as: disk drives (DASD), tape drives, terminals (ex: 3270s), printers, telecommunications gear, and so forth.

Much of the above is more succinctly summarized in the S/370 Reference Summary (REFSUM), see the Links section to get yours.

For those who need a refresher on Binary, Decimal, and Hexadecimal numbers, you might wish to review the Numbers page.