Zilog EZ80F916 User Manual
Page 26
eZ80
®
CPU
User Manual
UM007715-0415
Registers and Bit Flags
17
Half-Carry Flag (H)
The Half-Carry flag (H) is set or reset, depending on the carry and borrow status between
bits 3 and 4 of an 8-bit arithmetic operation. This flag is used by the decimal adjust accu-
mulator instruction (DAA) to correct the result of a packed BCD addition or subtraction.
The H flag is set to 1 or reset to 0, as indicated in
.
Zero Flag (Z)
The Zero flag (Z) is set to 1 if the result generated by the execution of
certain instructions is 0. For 8-bit arithmetic and logical operations, the Z flag is set to 1 if
the resulting byte in the accumulator is 0. If the byte is not 0, the Z flag is reset to 0.
For compare instructions, the Z flag is set to 1 if the value in the
accumulator is the same as the data it is being compared against. When testing a bit in a
register or memory location, the Z flag contains the
complemented state of the indicated bit (see the BIT b, r instruction.
When inputting or outputting a byte between a memory location and an I/O device (for
example, INI, IND, OUTI and OUTD), the B register is decremented. If the result of this
decrement is 0 (that is, B–1 = 0), then the Z flag is set to 1. Otherwise, the Z flag is reset
(cleared to 0). Also, for byte inputs from I/O devices using IN r,(C), the Z flag is set to 1 to
indicate a zero-byte input.
Sign Flag (S)
The Sign flag stores the state of the most significant bit of the
accumulator (bit 7). When the CPU performs arithmetic operations on signed numbers,
binary two’s-complement notation is used to represent and process numerical information.
A positive number is identified by a 0 in bit 7. A negative number is identified by a 1.
The binary equivalent of the magnitude of a positive number is stored in bits 0–6 for a
total range of 0–127. A negative number is represented by the two’s-complement of the
equivalent positive number. The total range for negative numbers is –1 to –128.
When inputting a byte from an I/O device to a register, IN r,(C), the S flag indicates either
positive (S = 0) or negative (S = 1) data.
Table 9. H Flag Settings
H
ADD
SUBTRACT
1 There is a carry from bit 3 to bit 4 There is a borrow from bit 4.
0 There is no carry from bit 3 to bit
4
There is no borrow from bit 4.