Rainbow Electronics ATAR862-3 - Manual (Page 16)

16
ATAR862-3
4556B–4BMCU–02/03
Figure 12. RAM Map
Registers The microcontroller has seven programmable registers and one condition code register
(see Figure 13).
Program Counter (PC) The program counter is a 12-bit register which contains the address of the next instruc-
tion to be fetched from the ROM. Instructions currently being executed are decoded in
the instruction decoder to determine the internal micro-operations. For linear code (no
calls or branches), the program counter is incremented with every instruction cycle. If a
branch-, call-, return-instruction or an interrupt is executed, the program counter is
loaded with a new address. The program counter is also used with the table instruction
to fetch 8-bit wide ROM constants.
Figure 13. Programming Mode l
RAM
FCh
00h
Autosleep
FFh
03h
04h
X
Y
SP
RP
TOS-1
Expression
stack
Return
stack
Global
variables
R
A
M address r
egister:
07h
(256 x 4-bit)
Global
variables
4-bit
TOS
TOS-1
TOS-2
3 0
SP
Expression stack
Return stack
0 11
12-bit
RP
v
TOS
CCR
0 3
0 3
0 7
0 7
7
0 11
RP
SP
X
Y
PC
-- B I
Program counter
Return stack pointer
Expression stack pointer
RAM address register (X)
RAM address register (Y)
Top of stack register
Condition code register
Carry / borrow
Branch
Interrupt enable
Reserved
0 7
C
0
0 0