Chapter 37 ½ Intel Microprocessors: 8008 to 8086 621
1. General Registers. The 8080 registers are the same seven 8-bit registers that were in the 8008 scratchpad-namely A,B,C, D,E,H, and L. In order to incorporate 16-bit data facilities in the 8080, certain instructions operate on the register pairs BC, DE, and HL.
The seven registers can be used interchangeably for on-chip temporary storage. The three register pairs are used for address manipulations, but their roles are not interchangeable; there is an 8080 instruction that allows operations on DE and not BC, and there are address modes that access memory indirectly through BC or. DE but not HL.
As in the 8008, the A register has a unique role in arithmetic and logical operations: it serves as one of the operands and is the receptacle for the result. The HL register again has its special role of pointing to the pseudo-register M.
2. Stack Pointer and Program Counter. The 8080 has a single program counter instead of the floating program counter of the 8008. The program counter is 16 bits (two bits more than the 8008's program counter), thereby permitting an address space of 64K.
The stack is contained in memory instead of on the chip, which removes the restriction of only seven levels of nested subroutines. The entries on the stack are 16 bits wide. The 16-bit stack pointer is used to locate the stack in memory. The execution of a call instruction causes the contents of the program counter to be pushed onto the stack, and the return instruction causes the last stack entry to be popped into the program counter. The stack pointer was chosen to run "downhill" (with the stack advancing toward lower memory) to simplify indexing into the stack from the user's program (positive indexing) and to simplify displaying the contents of the stack from a front panel.
Unlike the 8008, the stack pointer is directly accessible to the programmer. Furthermore, the stack itself is directly accessible, and instructions are provided that permit the programmer to push and pop his own 16-hit items onto the stack.
3. Flags. A fifth flag, AUXILIARY CARRY, augments the 8008 flag set to form the flag set of the 8080. The AUXILIARY CARRY flag indicates if a carry was generated out of the four low-order bits. This flag, in conjunction with a decimal-adjust instruction, provides the ability to perform packed BCD addition (see Appendix 2 for details). This facility can be traced back to the 4004 processor. The AUXILIARY CARRY flag has no purpose other than for BCD arithmetic, and hence the conditional transfer instructions were not expanded to include tests on the AUXILIARY CARRY flag.
It was proposed too late in the design that the PARITY flag should double as an OVERFLOW flag. Although this feature didn't make it into the 8080, it did show up two years later in Zilog's Z-80.
C. Instruction Set
The 8080 includes the entire 8008 instruction set as a subset, The added instructions provide some new operand-addressing modes and some facilities for manipulating 16-bit data. These extensions have introduced a good deal of asymmetry. Typical instruction formats are shown in Fig. 1. A summary of the 8080 instructions appears in Fig. 4.
The only means that the 8008 had for accessing operands in memory was via the M register. The 8080 has certain instructions that access memory by specifying the memory address (direct addressing) and also certain instructions that access memory by specifying a pair of general registers in which the memory address is contained (indirect addressing). In addition, the 8080 .includes the register and immediate operand-addressing modes of the 8008. A 16-bit immediate mode is also included,
The added instructions can be classified as load/store instructions, register-pair instructions, HL-specific instructions, accumulator-adjust instructions, carry instructions, expanded I/O instructions, and interrupt instructions.
The load/store instructions load and store the accumulator register and the HL register pair using the direct and indirect addressing mode. Both modes can be used for the accumulator, but due to chip size constraints, only the direct mode was implemented for HL.
The register-pair instructions provide for the manipulation of 16-bit data items. Specifically, register pairs can be loaded with
16-bit immediate data, incremented, decremented, added to HL, pushed on the stack, or popped off the stack. Furthermore, the flag settings themselves can be pushed and popped, thereby simplifying saving the environment when interrupts occur (this was not possible in the 8008).
The UL-specific instructions include facilities for transferring HL to the program counter or to the stack pointer, and exchanging HL with DE or with the top entry on the stack. The last of these instructions was included to provide a mechanism for (1) removing a subroutine return address from the stack so that passed parameters can be discarded or (2) burying a result-to-be-returned under the return address, This became the longest instruction in the 8080 (5 memory cycles); its implementation precluded the inclusion of several other instructions that were already proposed for the processor.
Two accumulator-adjust instructions are provided. One complements each bit in the accumulator and the other modifies the accumulator so that it contains the correct decimal result after a packed BCD addition is performed.
The carry instructions provide for setting or complementing the CARRY flag. No instruction is provided for clearing the CARRY flag. Because of the way the CARRY flag semantics are defined, the CARRY flag can be cleared simply by ORing or ANDing the accumulator with itself.