Software

Programming languages: see the summary in Table 6.1

**Table 6.1:** The hierarchy of programming languages
Language	Examples	Comments	C $^\dagger$
4th generation	code generators, AI	large projects, productivity
High-level	Fortran, Pascal, Basic	easy on humans, hw independent	$\bullet$
Low-level	Assembler (mnemonics)	hard, but full control of hw	$\bullet$
Machine	operation codes	``understood'' by the CPU
Hardware	bits, 0/5V voltage levels

$^\dagger$ C bridges the two layers; is a high-level language but allows low-level extensions.

Compilers: computer programs that translate the human-readable source code into machine-executable op-codes.

Op-codes: for example, about 300 in six functional groups for 8088/8086:

data transfer	`mov, in, out, xchg`
arithmetic	`add, sub, inc, dec, mul, div`
logical	`and, or, xor, not`
string manipulation
control transfer	`jmp, call`
processor control

8088/8086 registers: Table 6.2 shows a list of 14 internal (to CPU) memory locations with very fast access used to keep track of where in the program the CPU is. Each register is 16-bit wide. However, General Registers (see Table 6.2) can also be addressed in two 8-bit chunks (xL and xH for Low and High bits, respectively).

**Table 6.2:** 8086/8088 CPU registers
16-bit segment	high 8 bits	low 8 bits	comments
General registers:
AX	AH	AL	accumulator register
BX	BH	BL
CX	CH	CL
DX	DH	DL
Addressing registers:
SI	-	-
DI	-	-
BP	-	-
Control registers:
SP	-	-
IP	-	-
Flags $^\dagger$	-	-
Segment registers:
CS	-	-	Code Segment pointer
DS	-	-	Data Segment pointer
ES	-	-
SS	-	-

$^\dagger$ Flags: consists of individual bits which flag certain conditions.
E.g., bit ZF=on (Zero Flag) means the last instruction yielded a 0 as a result.

SEGMENT:OFFSET addressing: 16 bits can address 65,536 locations = size of segment; 20 bits can address 1MB

segment (16 bits)

0 0 0 0

offset (16 bits)

0 0 0 0

address (20 bits)

IBM PC memory map is summarized in Table 6.3

**Table 6.3:** IBM PC memory map
`00000 - 003FF`	I/O
`00400 - 0047F`	BIOS Ram
`00480 - 005FF`	RAM for special purposes (ROM Basic)
`00600 - 9FFFF`	Program memory
`A0000 - AFFFF`	VGA video expansion
`B0000 - B0FFF`	IBM monochome video
`B8000 - BFFFF`	CGA video memory
`C0000 - CFFFF`	reserved for video expansion
`D0000 - D7FFF`	ROM, usually not all installed
`D8000 - DFFFF`	"
`E0000 - E7FFF`	"
`E8000 - EFFFF`	"
`F0000 - F3FFF`	reserved for ROM
`F4000 - F5FFF`	"
`F6000 - FDFFF`	Basic in ROM in the original IBM PC
`FE000 - FFFFF`	BIOS ROM