文档介绍：Scanned and converted to PDF by HansO, 2001
Chapter 3 page 1-42
In this PDF: Chapter 3 page 1-42
CPU REGISTERS AND STATUS FLAGS
The CPU registers and status flags for the 280 may be illustrated as follows:
Zero
Auxiliary Carry
Pariry/OverNow
-Subtract
7 v T v
S 1 Z lAcl ]P/~] N 1
A
S-ndav { Secondary Data Counters
Accumulators
Ill}Primary Data Counter
S' Z' Ac' P/O, N' C' Alternate Flags
A' Alternate Accumulator
C'
Alternate B' Alternate Secondary
Data Counters
Secondan/ D' E'
Accumulators H' L' Alternate Pnmaty Data Counter
SP Stack Pointer
PC Program Counter
IX lndex Register
IY Index Reaster
I Interrupt Vector Register
1 R Refresh Reg~ster
The Accumulator is the primary source and destination for one-operand and two-
operand instructions. For example, the shortest and fastest data transfers between the
CPU and 1/0 devices are performed through the Accumulator. In addition, more Memo-
ry Reference ~nstructionsmove data between the Accumulator and memory than bet-
ween any other register and memory All 8-bit arithmetic and Boolean instructions take
one of the operands from the Accumulator and return the result to the Accumulator. An
instruct~onmust therefore load the Accumulator before the 280 can perform any 8-
bit arithmetic or Boolean operations.
The B, C, 0, E, H, and L registers are all secondary registers. Data stored in any of
these six registers may be accessed with equal ease: such data can be moved to any
other reglster or can be used as the second operand In two-operand instructions.
There are, however, some ~mportantdifferences in the functions of Reg~stersB. C. D. E.
H. and L
Registers H and L are the primary Data Pointer for the 280. That is to say. you will
normally use these two registers to hold the 16-bit memory address of data being ac-
cessed. Data may be transferred between any registers and the memory location ad-
dressed by H and L S