The MOS Technology 6508 is an

8-bit In computer architecture, 8-bit integers or other data units are those that are 8 bits wide (1 octet). Also, 8-bit central processing unit (CPU) and arithmetic logic unit (ALU) architectures are those that are based on registers or data buses of ...

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MOS Technology MOS Technology, Inc. ("MOS" being short for Metal Oxide Semiconductor), later known as CSG (Commodore Semiconductor Group) and GMT Microelectronics, was a semiconductor design and fabrication company based in Audubon, Pennsylvania. It is mo ...

. Based on the popular

6502 The MOS Technology 6502 (typically pronounced "sixty-five-oh-two" or "six-five-oh-two") William Mensch and the moderator both pronounce the 6502 microprocessor as ''"sixty-five-oh-two"''. is an 8-bit microprocessor that was designed by a small te ...

, the 6508 is augmented with two additional features: an internal 8-bit

digital Digital usually refers to something using discrete digits, often binary digits. Technology and computing Hardware *Digital electronics, electronic circuits which operate using digital signals **Digital camera, which captures and stores digital i ...

I/O port Memory-mapped I/O (MMIO) and port-mapped I/O (PMIO) are two complementary methods of performing input/output (I/O) between the central processing unit (CPU) and peripheral devices in a computer. An alternative approach is using dedicated I/O pr ...

and 256

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of internal

static RAM Static random-access memory (static RAM or SRAM) is a type of random-access memory (RAM) that uses latching circuitry (flip-flop) to store each bit. SRAM is volatile memory; data is lost when power is removed. The term ''static'' differe ...

Memory layout

The internal RAM is mapped into the CPU

address space In computing, an address space defines a range of discrete addresses, each of which may correspond to a network host, peripheral device, disk sector, a memory cell or other logical or physical entity. For software programs to save and retrieve ...

both at - and at -, so it can serve as both

zero page The zero page or base page is the block of memory at the very beginning of a computer's address space; that is, the page whose starting address is zero. The size of a page depends on the context, and the significance of zero page memory versus ...

and

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space. The I/O port is available at location , with a data-direction register at , which is the same layout of the 6510.

Variants

There may be different strapping/bonding variants of this chip; the datasheet below lists pin 40 as clock phase two, but in the CBM900 computer, the 6508 controlling floppydisk has pin 40 as "Set Overflow" (SO). SO is a pin that is also offered on the 40-pin 6502, but omitted on all the 28-pin 650x variants (6503–6507). The SO pin, on 6500 family CPUs that have it, sets the Overflow flag in the P register, which can be tested using BVC/BVS instructions. Using SO, a tight polling loop of 3 machine cycles can be constructed, tighter than the minimum two instructions of any test and branch loop not using the SO pin functionality.

References

External links

MOS 6508 datasheet (GIF format, zipped)
{{MOS CPU 65xx microprocessors MOS Technology microprocessors 8-bit microprocessors

Memory layout

Variants

References

Further reading

External links